Search

Dense sialon ceramics along the tie line between Si{sub 3}N{sub 4} and Nd{sub 2}O{sub 3}{center_dot}9AlN were prepared by hot-pressing at 1,800 C. The materials were subsequently heat-treated in the temperature range 1,300--1,750 C and cooled either by turning off the furnace (yielding a cooling rate (T{sub cool}) of {approx} 50 C/min) or quenching (T{sub cool} {ge} 400 C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase {alpha}-sialon forms for 0.30 {le} x {le} 0.51 in the formula Nd{sub x}Si{sub 12{minus}4.5x}Al{sub 4.5x}O{sub 1.5x}N{sub 16{minus}1.5x}. The {alpha}-sialon is stable only at temperatures above 1,650 C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an {alpha}-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed {alpha}-Sialon decomposes to the more stable {beta}-sialon and either the melilite phase or a new phase of the composition NdAl(Si{sub 6{minus}z}Al{sub z})N{sub 10{minus}z}O{sub z}. Nd-doped {alpha}-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22.5 GPa) and a fracture toughness (K{sub IC} = 4.4 MPa{center_dot}m{sup 1/2}) at room temperature. The presence of themore » melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.« less

Duplex α,β-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350-1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R 2 Si 3-x Al x O 3+x N 4-x , β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness (H v10 ≃ 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPa.m 1/2 ). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPa.m 1/2 ).

A tribological study has been carried out on a new type of carbide-metal composite, in which the two phases form a continuous skeleton microstructure interwoven throughout the body. The composites' resistance to two-body abrasion is evaluated in a pin-on-drum set-up with diamond and SiC abrasive papers. Also sliding wear and friction tests with steel and alumina as counter materials were undertaken. The composites show promising tribological properties, comparable to those of established wear resistant materials. Scanning electron microscopy was used to study the worn surfaces. The implications of the skeleton microstructure on the abrasive wear resistance are discussed on the basis of a wear model for multiphase materials.

Nanoporous carbons have been prepared through a selective etching reaction, performed by halogenisation of aluminum carbide. The structures obtained can be controlled by varying the chlorination temperature. The unique set of nanocarbons obtained include nanoporous amorphous structures, the nanobarrel structure and a nanographitic structure as chlorination temperatures were increased from 400 °C to 1000 °C. The synthesis process gives a pure product with high yield, and may be scaled up to produce bulk amounts.

We have investigated the phase composition of lithiated samples of nanoporous carbons (NPCs) obtained from carbides (SiC, TiC, Mo 2 C) by chlorination. Lithium insertion was produced at the temperature range 100-200 °C via vacuum deposition and diffusion. The effect of the deposition and diffusion duration on the phase composition has been studied. Phase composition was found to be dependent on the relationship between deposition and diffusion rates. The condition for producing the samples uniformly impregnated by lithium and containing intercalation phases only was determined.

In this work, the results of a study of the lithium insertion process in nanoporous carbons (NPC) obtained from carbides (SiC, TiC, Mo2C) by chlorination are presented. Lithium insertion was produced in the temperature range 30–200 °C via vacuum deposition and diffusion. The major analytical tools for study included nuclear reaction method and X-ray diffraction. Diffusion coefficients of lithium at this temperature interval were estimated. A possible mechanism of lithium diffusion is discussed.

A technique is proposed for fabricating diamond–SiC–Si composites via chemical reactions between silicon and carbon directly in the green compact at pressures no higher than 105Pa. An important advantage of the process is the ability to fabricate complex-shaped bodies from diamond-containing composites exhibiting unique mechanical properties.

The erosion and abrasion resistances of a new type of composite consisting of diamond particles embedded in a silicon carbide matrix have been evaluated. The investigation is a comparative study in which a conventional cemented carbide, a commercially available polycrystalline diamond material and some common engineering ceramics are included as reference materials. The materials are characterised in terms of wear rates and scanning electron microscopy images of the worn surfaces. It was found that the new diamond–silicon carbide composites were comparable to the commercial diamond composite, and out-competed the other reference materials in erosive and abrasive wear. The results are discussed in terms of the microstructural characteristics of the composites.

Microstructures of the 10 vol% TiN particulates added Y-doped α -SiAlON ceramic composites were studied by using TEM, HREM and EDS techniques. α -SiAlON and TiN were the only main phases in this fully dense material. Less than 1 vol% amount of glass was detected and the majority of the glass was located at the triple grain boundary junction nodes while the boundaries remained clean. No reaction was noticed between the α ′ matrix and TiN particles, but some TiN particles, included within the matrix grains, were epitaxial with α -SiAlON. The extensive microcracking commonly existing around the very large TiN grains, due to the high thermal expansion mismatch between α ′ and TiN, was considered to have brought about negative effects on the strength of the composite materials and on toughening. The porosity and agglomeration of TiN particles resulting from local inhomogeneity of the liquid during the sintering process further decreased the mechanical properties of the composites as they acted as cracking sources. Finally, possible processes were discussed for the improvement of the properties of α -SiAlON/TiN composites.

Simultaneous carboreduction and nitridation (CRN) of Y2O3 doped SiO2 /clay or elemental Si/clay mixes at atmospheric pressure were used to prepare low-cost powders of α-sialon composition. The synthesised products contained crystalline phase mixtures of α- and β-sialon and Y3Si6N11 . Subsequent hot pressing of the CRN powders at 1800 °C resulted in poorly densified materials from mixtures with low Y2O3 , whereas high Y2O3 gave dense ceramics. The addition of SiO2to the starting mix resulted in ceramics containing substantial amounts of very fine SiC particles distributed in a matrix of β-sialon and glass. Solid state MAS NMR indicates that the crystalline SiC observed by XRD after hot-pressing is also present in an XRD-amorphous form in the pre-pressed powders, and that hot-pressing causes the Al–N units to become overall more oxygenated, with the conversion of β-sialon to α-sialon and the formation of small amounts of polytypoid sialons. The hardness,HV10 , and fracture toughness, K1C , of the densified materials were ca. 18 GPa and 3 MPa m1/2 , respectively. The use of elemental Si produced less SiC, the major phase in these ceramics being α-sialon and a residual glassy phase observable by electron microscopy. The hardness and toughness of these densified materials was ca. 20 GPa and 3 MPa m1/2 , respectively.

α-SiAlON samples of the overall composition SmxSi12−(m+n)Alm+nOnN16 − m, with m and n in the ranges 0.6 ≤ m ≤ 1.44 and 1.3 ≤ n ≤ 1.7 (m = 3x), were prepared by a hot-pressing technique at 1800 and 1700 °C. Based on X-ray powder diffraction studies and elemental analysis of individual α-SiAlON grains in the obtained ceramic compacts, the extension of the Sm-doped α-SiAlON solid solution range was mapped out. The m-value was found to vary between 0.89 and 1.52 while n was always less than 1.23. Elongated α-SiAlON grains were found preferentially in compacts having overall compositions located slightly outside the oxygen-rich border-line of the homogeneity region of the Sm-doped α-SiAlON phase, i.e. for m ≈ 1.2 and n ≈ 1.3. We also noticed that elongated α-SiAlON grains were formed preferentially perpendicular to the pressure applied in the sintering procedure. All samples exhibited HV10-values in the range 21–22 GPa and KIC-values in the range 4.0–4.7 M Pa m 1 2 .

Duplex Y-, Yb- and Nd-doped α-β-sialon ceramics were fabricated from starting powder compositions within the two-phase α′-β′ area. The microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix and only smaller volume fractions of intergranular phases. The morphology of the intergranular regions was determined by the overall chemistry and the doping element. The β′ a′+β′ weight ratio as well as the α and β-sialon substitution levels were dependent upon the α′ stabilizing cation. Sialons with an β′ α′+β′ weight ratio of around 0.5 had a favourable combination of high hardness, HV10 around 21 GPa, and indentation fracture toughness, 5.0–5.5 MPa m 1 2 . The toughness of these duplex sialons was however reduced to below 4 MPa m 1 2 after long-term heat treatment at 1450 °C. The α′ stabilizing elements Y and Yb resulted in α-sialon structures which were stable during heat treatment at 1450 °C, while the Nd α′ structures decomposed into Al-substituted melilite, Nd2Si2AlO4N3, β-sialon and the 21R sialon polytype at this temperature. The results imply that duplex α-β-sialons stabilized by Y and Yb have a high potential as engineering ceramics.

A method is described for synthesising complex silicon lanthanide nitrides by carbothermal reduction and nitridation (CRN) of both SiO 2 –Ln 2 O 3 mixtures and elemental Si–Ln 2 O 3 mixtures at 1400–1450 °C. Syntheses in the Si 3 N 4 –YN system confirm the existence of the compounds YSi 3 N 5 and Y 2 Si 3 N 6 , which can be prepared virtually free of oxygenated impurity phases from mixtures containing SiO 2 rather than from mixtures containing elemental Si. A third compound was also prepared in this system, with an XRD pattern corresponding to that reported for Y 6 Si 3 N 10 ; mass change data and EDX analyses suggest, however, that this phase is much less Y-rich than Y 6 Si 3 N 10 , with an apparent composition nearer Y 3 Si 6 N 11 . X-Ray powder diffraction data and 29 Si and 89 Y solid-state MAS NMR spectra are reported for all three phases. The NMR results suggest the presence of at least three Si sites in each phase, and at least two types of Y site, one of which, characterised by a resonance at δ 490–510 (ref. YCl 3 ), is common to all three compounds.

Nucleation and growth behavior of the elongated α′-SiAlON grains have been studied in α′-SiAlON ceramic with an overall composition Y 0.48 Si 10.00 Al 2.30 O 1.17 N 15.29 prepared by hot-pressing at 1825 °C. Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have shown that the elongated α′-SiAlON grains always contain an α-Si 3 N 4 core, implicating heterogeneous nucleation operating in the present system. The growth mode is epitaxial. Selected area diffraction (SAD) and HREM analyses reveal that the crystal structure of the elongated α′-SiAlON has dilated and does not strictly comply with the hexagonal construction, i.e. the c axis tilting by no more than 2 ° is no longer normal to (001) plane, giving a conclusion that the deformed structure, probably formed by local high stress at high temperatures, changes the chemical bonding conditions especially on (001) plane, leading to faster deposition of atoms on (001) plane than on other planes. Additionally, morphology and growth defects have been studied in this paper.

Dense ceramics with overall compositions DyxSi12-4.5xAl4.5xO1.5xN16-1.5x, where 0.2≤x≤1.0, along the Si3N4–Dy2O3·9AlN tie line were prepared by hot-pressing at 1800°C. The dysprosium-doped α-sialon phase formed in the composition range 0.3≤x≤0.7. Sintered materials of different compositions were post-heat-treated at temperatures in the range 1300–1750° C for different times and it was shown that the Dy-α-sialon phase is stable over a large temperature interval and during heat treatment times up to 30 days. Unlike corresponding neodymium- and samarium-doped α-sialons, dysprosium-doped α-sialon does not decompose into β-sialon and rare-earth-rich grain-boundary phase(s) at temperatures below 1550°C. The α-phase can coexist with a liquid phase at temperatures ≥1550°C and with the Dy-M′-phase (Dy2Si3-xAlxO3+xN4-x) at lower temperatures. When heat treated at 1450°C, any residual liquid grain-boundary phase reacted with minor amounts of the α-sialon phase and devitrified to Dy-M′-phase, yielding a glassy phase-free material. The Dy-M′-phase formed had the maximum aluminium substitution, i.e. x≈0.7. Dysprosium-doped α-sialon exhibited very high hardness (Hv10=22 GPa) and a fracture toughness of 4.5 MPa m1/2, and the hardness and toughness decreased only slightly after devitrification of the glassy phase. Some elongated α-sialon grains were formed at high x values in glassy phase-containing materials, but their presence did not affect the toughness significantly.

Carbothermal formation of low-z β′-sialon (z=0.5) from halloysite clay has been carried out using two different forms of the necessary additional silicon, namely, finely divided SiO 2 (quartz) and elemental Si. XRD and solid-state 29 Si and 27 Al MAS NMR studies of the early stages of the reaction at 1000–1400 °C show that the SiO 2 forms both Si 3 N 4 , via a series of oxynitrides, and SiC. These then react with the mullite produced by thermal decomposition of the clay, to form β′-sialon. The addition of 3 mass% Y 2 O 3 to the reaction mix does not change the reaction sequence, but facilitates the formation of both mullite and β′-sialon, in which it also promotes the formation of Al–N units. Using elemental Si, some SiC is formed, which reacts with the mullite and remaining Si to form O′-sialon, which may then react further with mullite to produce β′-sialon. The addition of Y 2 O 3 forms transient Y 2 Si 2 O 7 at lower temperatures, but in the later stages facilitates the formation of β′-sialon with respect to O′-sialon.

Dense, Yb-doped -sialon ceramics of the composition , with , together with some Yb-based - and -sialon ceramics containing an excess of glassy phase, have been prepared by hot-pressing at . These materials were subsequently heat-treated in the temperature range for different periods. It was found that Yb-doped -sialon forms within a wide composition region, with x ranging from 0.3 to more than 1.0, and that -sialon is stable over a large temperature interval and during heat treatment times of up to 30 days. The -sialon phase coexists with a liquid phase at and with crystalline phases such as the Yb garnet and/or J phase at . Further heat treatment at revealed that initially (during the first 24 h) the -sialon phase reacts with the residual liquid grain-boundary phase, yielding a material which contains an Yb-stabilized garnet phase, besides the -sialon phase, but no glassy phase. This phase assembly is stable upon further heat treatment (30 days). Samples containing Yb-doped -sialon in conjunction with an amorphous intergranular phase show very high hardness and fracture toughness in the range of at room temperature, and mechanical data in the same range were obtained for samples exposed to prolonged heat treatment at . Elongated -sialon grains were found in samples with high x values, but their presence did not affect the mechanical properties significantly.

To a powder mixture of an overall α-sialon composition R 0.4 Si 10.2 Al 1.8 O 0.6 N 15.4 , with R = Nd, Sm, Dy and Yb, were added extra amounts of powder mixtures, (20%), having Si:Al:R and O:N atomic ratios of 2:1:1 and 3:1, respectively. Series of α-rich mixed α/β-sialon ceramics containing about 20–30 vol% glassy phase were prepared from these powders by pressureless sintering at 1750 °C. The as-prepared samples were subsequently heated at 1750 °C and then quenched to room temperature or to 1150, 1300 and 1450 °C (with a cooling rate exceeding 400 °/min), and were annealed at these temperatures for various times. The samples quenched to room temperature revealed that α-sialon coexists with β-sialon and a liquid phase at 1750 °C. The post heat-treatment at the lowest temperature involved a devitrification of the glassy phase and resulted in mixtures of mainly rare earth oxynitrides like the U-phase R 3 Si 3 Al 3 O 12 N 2 , wollastonite R SiO 2 N or the B-phase Dy 2 SiAlO 5 N. Post heat-treatment at 1450 °C induced a reaction between residual liquid and α-phase in the Nd- and Sm-systems and yielded a mixture of an oxygen- and a nitrogen-rich phase in all systems. Thus the melilite phase, R 2 Si 3−x Al x O 3+x N 4−x , is formed with all rare earth elements except Yb, which yields Yb 4 Si 2 O 7 N 2 . The oxygen-rich phase in the Nd- and Sm-systems was the aluminate R AlO 3 , while in the Dy- and Yb-systems the garnet phase, R 3 Al 5 O 12 , was formed. Similar results were obtained with samples quenched to 1300 °C. These findings suggest that the stability of α-sialon is related to the type of sintering aid used. The phase assemblage found in the as-prepared samples is discussed in view of the findings obtained in the annealing experiments.

Fully dense Si2N2O-ZrO2 composites have been prepared by glass-encapsulated hot isostatic pressing at 1600 and 1700 °C by two different routes. Firstly, from reacting powder mixtures of equimolar amounts of Si3N4 plus SiO2 with 0, 5 and 20 wt% ZrO2 or ZrO2 (3 mol% Y2O3), and secondly by an in-situ reaction from equimolar mixtures of Si3N4 and ZrSiO4 to form a Si2N2O composite with 36 wt% ZrO2. The effects of small additions of A12O3 or Y2O3 in the two different preparation routes were also studied. It was found that the reaction of Si3N4 with SiO2 was strongly influenced by the addition of sintering aids and temperature, whereas the reaction with ZrSiO4 was mainly temperature controlled and proceeded rapidly at 1700 °C. Addition of zirconia or stabilized zirconia did not appear to have any positive effect on the toughness, and the materials prepared were in general brittle with a fracture toughness of ~3 MPa m 1 2 . The observed hardness was typically 12–14 GPa for the composites. The composites which consisted of more or less unreacted starting materials had a very finegrained microstructure. In the fully reacted samples which had the highest fracture toughness the zirconia grains had increased in size compared to the more brittle samples.

Simultaneous carboreduction and nitridation (CRN) of clays provides a useful route for the preparation of low-cost β-sialon powders. During the course of the CRN reaction at 1400–1450 °C, mass losses of ca. 10% in excess of the theoretical amounts are typically found. This discrepancy is mainly due to transport of gaseous SiO from the reactants into the N2–CO gas stream, followed by condensation in the cooler parts of the furnace system. In this work, the condensed material lost from the reacting system was collected under controlled conditions, and its constitution studied by 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and complementary X-ray powder diffraction (XRD). Predictions from thermodynamic calculations of the various possible gas-phase reactions are compared with the observed results.

Dense Sm-doped α-sialon ceramics along the tie line Sm x Si 12-4.5x Al 4.5x O 1.5x N 16-1.5x between Si 3 N 4 and Sm 2 O 3 .9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated at different temperatures in the range 1300-1750°C for different times. The samples were either cooled by turning the furnace off, yielding a cooling rate (T col )≃50°C/min or quenched (T col ≥400°C/min) by quickly moving the samples to a connected cooling chamber. It was found necessary to apply this quenching technique in order to reveal the true phase relations at high temperatures. It was found from phase analysis of quenched samples that the α-sialon phase is formed for 0.3

MoSi{sub 2}-particulate-reinforced {alpha}-SiAlON ceramic composites 10, 20, 25, and 30 vol% were prepared by at 1,750--1,800C. The {alpha}-SiAlON matrix was of the composition (Y{sub 0.48}Si{sub 10.00}Al{sub 2.30}O{sub 1.17}N{sub 15.29}). The hardness for the fully dense samples changed from HV10 = 22.5 to 15.3 GPa and the toughness from 3.2 to around 5.2 MPa{center_dot}m{sup 1/2} when up to 30 vol% MoSi{sub 2} was present. Two interesting microstructural features have been found. First, with an increasing amount of MoSi{sub 2} a pronounced coalescence of MoSi{sub 2} particles formed a ``dual phase`` material. The second effect was the growth of elongated {alpha}-SiAlON grains in the matrix with 10 vol% MoSi{sub 2} added. The oxidation resistance has been determined, to be unaffected by the addition of 20 vol% MoSi{sub 2} at 1,250 C in oxygen gas of 1 atm pressure.

A powder mixture of α-Si 3 N 4 , AlN and Y 2 O 3 corresponding to an yttrium α-sialon composition, Y 0.4 Si 10.2 Al 1.8 O 0.6 N 15.4 , was sintered both in a high temperature X-ray diffraction unit and in a regular sintering furnace. X-ray analysis was performed and it was shown that high temperature X-ray diffraction can be used to monitor the kinetics of the α-sialon phase formation during sintering at temperatures between 1450 and 1580 °C as the reactions take place. A variety of yttrium-rich intermediate and secondary phases are formed during sintering; those formed in the HT-XRD unit were not the same as those formed in the regular sintering furnace, but they do not significantly influence the overall reaction sequence or the amount of α-sialon formed. Quantitative analysis of the acquired data was used to evaluate the kinetics of the α-sialon phase formation; the estimated activation energy of the initial stage α-sialon formation was 330 kJ mol . After this initial stage of relatively rapid α-sialon formation the process became inhibited.

A new nitrogen-rich phase has been observed to form in rare-earth-metal stabilised α-sialon preparations containing Sm and Nd. The X-ray powder pattern of this phase overlapped to a great extent that of α-sialon. Large quantities of this phase were also found in samples of the overall composition Ln2Al4Si11N18O4 with Ln = Ce and La. The La compound was selected for crystal structure determination, and the obtained X-ray pattern could be indexed with an orthorhombic unit cell; a= 9.4303(7), b= 9.7689(8), c= 8.9386(6)A. A structural model was refined in the space group Pbcn using the Rietveld method. Based on 267 theoretical reflections in the 2θ range 10–82°, the following R-values were obtained for the composition LaAl(Si6 –zAlz)N10 –zOz with z= 1; RWp= 1.8, R1= 6.6 and RF= 5.1 %. The Al atoms and the (Si, Al) atoms are tetrahedrally coordinated by (N,O) atoms, yielding an Al(Si,Al)6(N,O)103– network. The La atoms are found in tunnels extending along the [001] direction and are irregularly coordinated by seven (N, O) atoms. The structure of this phase contains structural fragments which show strong similarities with some present in LaSi3N5; their compositional relationship is evident if their chemical formulae are expressed as LaN–AlN–2(Si,Al)3(N,O)4 and 2LaN–2Si3N4.

[beta]-Sialon and Nd[sub 2]O[sub 3]-doped [alpha]-sialon materials of varying composition were prepared by sintering at 1,775 and 1,825 C and by glass-encapsulated hot isostatic pressing at 1,700 C. Composites were also prepared by adding 2--20 wt% ZrO[sub 2] (3 mol% Nd[sub 2]O[sub 3]) or 2--20 wt% ZrN to the [beta]-sialon and [alpha]-sialon matrix, respectively. Neodymium was found to be a fairly poor [alpha]-sialon stabilizer even within the [alpha]-phase solid solution area, and addition of ZrN further inhibited the formation of the [alpha]-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO[sub 2](Nd[sub 2]O[sub 3]) or ZrN were seen in both the HIPed [beta]-sialon/ZrO[sub 2](Nd[sub 2]O[sub 3]) composites and the HIPed Nd[sub 2]O[sub 3]-stabilized [alpha]-sialons with ZrN additions.

Fully dense composites of 0–30 wt% discrete TiN particles distributed in a s-sialon matrix of overall composition Si 5·5 Al 0·5 O 0·5 N 7·5 have been prepared by hot isostatic pressing at 1650 and 1750°C. Pressureless sintering at 1775°C gave materials with an open porosity. Typical sizes of the TiN particles were 1–3 μm, and no intergranular glassy phase was observed in the prepared materials. The grain size of s-sialon was below 1 μm in the materials HIPed at 1650°C, and 1–2 μm at 1750°C. The Vickers hardness was fairly constant for the TiN-s-sialon composites with up to 15 wt% TiN added: H v 10 around 17·5 GPa for materials HIPed at 1650° and around 17 GPa at 1750°C, whereas at higher TiN contents the hardness decreased to around 16 GPa. The indentation fracture toughness of the s-sialon ceramic increased approximatively from 3 to 4 MPam 1 2 at an addition of 15 wt% TiN particulates. The fracture toughness could be further increased to 5 MPam 1 2 by addition of small amounts of Y 2 O 3 and A1N to a s-sialon composite with 30 wt% TiN.

Multi-phase SiAION ceramics offer great scope for property design by controlling the types of phases present and the microstructure development. The most common SiAION composites are the pressureless sintered, glassy-phase containing β or α-β SiAION ceramics, where the presence of α SiAION acts as a means of controlling the hardness and the growth of elongated β SiAION crystals in the microstructure improves the toughness. The SiAION ceramics can also be reinforced by separate additions of other hard, refractory compounds such as SiC and TiN. The best effect is obtained if different toughening mechanisms are combined. The phase composition and microstructure of SiAION composites are discussed and related to some observed mechanical properties.

Typical β-sialon and mixed α-β-sialon starting compositions have been densified by pressureless sintering using either Ln2O3 or equimolar mixtures of Ln2O3 + Y2O3 where Ln = Sm, Dy, Yb. The resulting materials were heat treated at 1000–1500°C to devitrify the grain boundary glass into crystalline oxynitride phases. The effects of rare earth ionic size and heat-treatment temperature on the stability of devitrified phases, microstructure and mechanical properties have been studied. Microstructural characterisation of the α-sialon phase formed in these systems revealed that α is only stable at high temperatures and transforms to β-sialon plus grain boundary phases at lower temperatures; this provides an excellent mechanism for controlling the mechanical properties of the final material.

Sialon ceramics have been prepared from two different Si3N4 powders obtained from different commercial sources, using Y2O3 and/or Nd2O3 as sintering aid. Although both powders were high-purity and high-quality silicon nitrides, surprisingly different phase compositions were obtained for the same overall starting compositions upon sintering at 1825°C. The phase compositions, densities, microstructures and mechanical properties of the samples obtained from the two powders have been compared. The samples prepared from one of the powders were oxidized at 1350°C for 20 h in dry oxygen in a TG unit. The oxidation curves obtained have been interpreted within the framework of a new rate law. The new rate law covers the observation that the oxidation behaviour of Si3N4-based ceramics is often non-parabolic, probably as a result of crystallization processes and the formation of cracks and/or bubbles within the oxide scale or at the scale/matrix interface during oxidation.

O′-sialon ceramics, Si2 − xAlxO1 + xN2 − x, with x = 0−0·2 have been prepared from Si3N4, SiO2 and Al2O3 by glass-encapsulated hot isostatic pressing at 1600, 1750 and 1900°C and are compared with materials prepared by pressureless sintering at 1600 and 1775°C. Additional series were prepared with 1,5 or 10 wt% Y2O3 as a sintering aid. Formation of the O′-sialon phase during sintering was dependent on the presence of a liquid phase and was promoted by alumina or yttria additions, especially when added together. Single-phase O′-sialon ceramics were never found, as small amounts of Si3N4 were always present. The microstructure consisted mainly of large elongated plate-like O′-sialon grains. Inclusions of β-Si3N4 were found in the small grains, which caused a significant amount of defects. Dense O′-sialon materials were hard (HV 10 about 1500) and brittle (K1C about 3·5 MPa m 1 2 ) at room temperature. The generally low toughness might be caused by the numerous defects in the Si2N2O structure in combination with a strong bond between the grains and the intergranular phase.

Alumina composites have been prepared with particle inclusions of 0–30 wt% titanium carbonitride and/or 0–5 wt% nickel, or nickel plus molybdenum metal. The metal was added in three different ways; as metal powder, as metal oxide, or as the intermetallic compound Ti2Ni. Pressureless sintering at 1750°C gave densities varying from 90% of theoretical density to full density. All materials were post-HIPed to full density at 1600°C before measurement of mechanical properties. Addition of metal alone increased the fracture toughness from 3·0 to 3·7 MPam 1 2 , but decreased the Vickers hardness, HV 10, from 1650 to 1500. The simultaneous addition of hard titanium carbonitride inclusions compensated for the decrease in hardness and gave a further increase in fracture toughness. The alumina composites with 5 wt% metal and 30 wt% Ti(C,N) inclusions had a hardness of 1800 and a fracture toughness of about 5 MPam 1 2 .

Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Yx(Si12–4.5x, Al4.5x)-(O1.5x,N16–1.5x) with 0.1 ≤x≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon (x= 0.1) to large amounts of α-sialon (x= 0.4) and increasing amounts of mellilite and sialon polytypoids (x= 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x= 0.3 and x= 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness (KIC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.

The reaction between AlN and small amounts of the oxides MgO, TiO2, Cr2O3, Fe2O3, Y2O3, ZrO2 Nb2O5 or Ta2O5 has been studied at temperatures of 1820°C and 1950°C. All reactants were embedded in BN powder and heated in a graphite furnace under a nitrogen atmosphere. At 1820°C MgO, TiO2, Cr2O3, Fe2O3, Nb2O5 and Ta2O5 reacted to form the nonstoichiometric oxynitride spinel phase ≈ Al23O27N5 and Mg3N2, Ti(C, N), CrB, Fe2B, NbC or TaC. At 1950°C aluminium oxynitride polytypoids were formed in place of the spinel. These possessed a needle-like morphology in the AlN-MgO, AlN-TiO2 and AlN-Cr2O3 systems and an equiaxed morphology in the AlN-Fe2O3 and AlN-Nb2O5 systems. The AlN-ZrO2 samples reacted to form only Zr(C, N) and the AlN-Y2O3 samples reacted to form only Al2Y4O9.

Nanoporous carbon prepared by chlorination of TiC powder has been characterized by high-resolution transmission electron microscopy (HRTEM) and showed a disordered amorphous structure of graphite planes with areas of an onion-like structure, where the graphite planes are ordered with concentric spherical graphitic shells (15–35 nm) interpenetrating and overlapping each other.

Si3N4-based ceramic materials formed by glass encapsulation and hot isostatic pressing (HIP) using different additions of Al2O3, Y2O3 and ZrO2 have been characterized by analytical electron microscopy and X-ray diffractometry. The microstructures have been related to formation process and to room temperature hardness and fracture toughness of the ceramics. A high volume fraction of retained α-Si3N4 after processing at 1550 °C gave the Si3N4 ceramics high hardness. The equi-axed grain morphology of the Si3N4 matrices in these materials, which contained only small amounts of residual glass, resulted in comparatively low fracture toughness values. Processing at 1750 °C reduced the amount of retained α-Si3N4 substantially. When Y2O3 was added, the microstructure contained a comparatively large volume fraction of residual glass, and the Si3N4 was present mainly as high aspect ratio β-Si3N4 grains. This type of microstructure gave an Si3N4 ceramic material with high fracture toughness combined with a lower hardness. Additions of ZrO2 and/or Al2O3 resulted also at 1750 °C in an extremely small volume fraction of residual glass, and a major part of the Si3N4 was present as equi-axed grains. These ceramics exhibited medium hardness and toughness values, however, larger additions of ZrO2 appeared to slightly increase toughness.

The reaction of AlN with Al2O3, SiO2 or Si3N4 and Al2O3 at temperatures of 1820°C and 1950°C and for compositions rich in AlN have been investigated using powder X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. At 1820°C, Al2O3 reacted with AlN to form the non-stoichiometric oxynitride spinel phase (≈Al23O27N5), whereas samples on the AlN-SiO2 and AlN-‘X-phase’ tie-lines did not react. At 1950°C, AlN-Al2O3 mixtures reacted to form the 27R aluminium oxynitride polytypoid, AlNSiO2 mixtures reacted to form 21R and 27R sialon polytypoids and AlN-‘X-phase’ mixtures reacted to form the 12H, 21R and 27R sialon polytypoids, all of which possessed a needle-like morphology. Ordered and disordered intergrowths were also observed in the polytypoid crystals but the disordered 2Hδ sialon phase was not found.

Si3N4-ZrO2 composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO2 and ZrO2 stabilized with 3 mol% Y2O3. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al2O3 and 0–6 wt% Y2O3. Composites prepared at 1550 °C contained substantial amounts of unreacted α-Si3N4, and full density was achieved only when ⩾ 1 wt% Al2O3 or ⩾ 4 wt % Y2O3 had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO2 starting powder was stabilized by Y2O3, fully dense Si3N4-ZrO2 composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm−2 and 4.5 MPam1/2, respectively. Examples of such materials were either Si3N4 formed with an addition of 2–6 wt% Y2O3 or Si3N4-ZrO2 composites with a simultaneous addition of 2–6 wt%Y2O3 and 2–4 wt% Al2O3.

Mixtures of Si3N4, SiO2, Al2O3 and AlN have been used to prepare series of specimens heat treated at 1700, 1730, 1750 and 1775°C. The phase relations of the liquid-rich area of the SiAlON system were studied in detail by X-ray diffraction and scanning electron microscope analysis. Each specimen was chemically analysed after the heat treatment to determine any changes in the overall composition. Finally, the observations have been used to obtain a slightly revised phase diagram at a temperature range of 1700–1730°C for the system Si3N4-SiO2-Al2O3-AlN.

α-β-Sialon materials were prepared with a constant molar addition of Y2O3 + CeO2 sintering aid with varied composition: 0, 25, 50, 75 and 100 mol% Y2O3. The α-sialon grains formed in the microstructure were carefully investigated by analytical and high-resolution electron microscopy. It was found that the α-sialon structure can accept a small amount of cerium to enter the large interstitial cages, if it is stabilized by simultaneous additions of yttrium. Extended crystal defects were found to occur in the α-phase framework; this has not been observed with only yttrium added.

Carbide-metal composites, such as Cr3C2-Cu, Cr3C2-bronze and TiC-Cu, were prepared by infiltrating liquid metal into carbide skeletons with open micro-porosity at high temperatures. X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectrometry were used to characterize these materials. The resulting microstructures consisted of two interpenetrating three-dimensional skeletons. There is no sign of porosity at the interface indicating good wetting between the metal and ceramic. The combination of a ceramic and a metal in the composites resulted in unique combinations of mechanical and electrical properties. Abrasion and friction tests showed that the composite materials had better abrasion resistance than that of ceramics such as Al2O3 and SiC as well as lower friction coefficients than that of SiC. Tests in electrical contacts have revealed a contact resistance similar to copper. The composites are well suited for special applications, such as sliding electrical contacts, arc resistant switches, friction components and bearing/seal components.

Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y2O3 and/or Ce02 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/CeO2: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y2O3 and pure CeO2, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the α sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the β sialon type of materials. For the mixed α-β sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping.

The hot hardness of polycrystalline single-phase α- or β- sialon ceramics declines with increasing temperature, but the measured Vickers hardness (HV1) at 1100°C is still about 1550 and 1300 for the α-sialon and the low-substituted β-sialon materials, respectively. The hardness of ‘composite’β- or α-β-sialon ceramics containing a high volume fraction of glassy phase is lower at all temperatures and drops significantly above about 900°C.

The phases occurring in the TiWO system close to WO3 at 1273 and 1373 K were determined using X-ray diffraction and electron microscopy. In samples heated for up to 2 weeks an extended phase field of {001} crystallographic shear structures was found to exist between MO2.88 and MO2.93 along the TixWO3 line. These CS structures intergrew with {103} CS structures in the composition region between TixWO3 and the binary WO line to form an operationally nonstoichiometric CS phase region. A disordered structure related to W12O34 with an extended homogeneity range was found for compositions TixW1−xO2.83 with 0 < x < 0.04. This phase was surrounded by a composition region in which disordered PC units were dispersed in the WO3-like matrix to form another nonstoichiometric phase region. Intergrowth between these PC structures and both CS phases and W18O49 were encountered. Heating for up to 2 months caused the ternary {001} CS and (TixW1−x)12O34 phases to slowly decompose and their apparent phase ranges to narrow.

On etudie le frittage a 1775 o C de Si 3 N 4 avec addition de 5 a 20% en poids de ZrO 2 ou de Y 2 O 3 -ZrO 2 et addition de petites quantites de Al 2 O 3 et/ou de Y 2 O 3 . On examine les teneurs en phases, la densite et les proprietes mecaniques des composites obtenus

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y2O3 and/or La2O3 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/La2O3:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreactedα-Si3N4 was present in the samples in addition toβ-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favouredα-sialon formation. The amount of intergranular phase increased by about 50% when Y2O3 was replaced by La2O3. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam−1/2, but the Vicker's hardness is slightly lower, being 1400 kg mm−2 at a 98 N load.

Sialon ceramics have been prepared by pressureless sintering at 1775 and 1825°C, using mixtures of Y 2 O 3 Nd 2 O 3 as sintering aids. It was found that at 1775°C less dense materials were obtained, but by raising the sintering temperature to 1825°C fully dense materials could be produced, even when pure neodymia was used. The hardness of the Nd-sialon ceramics was found to be slightly lower than for the corresponding Y-sialon ceramics, but the fracture toughness was approximately the same. It was also observed that the amount of intergranular phase increased when yttria was replaced by neodymia.