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Cermet coatings are one of the best surface protection of machine elements against wear. On the other hand, the most universal and economically justified method of applying such coatings is high velocity oxy-fuel (HVOF) spraying. This method makes it possible to produce coatings characterized by compact structure, low porosity and very good adhesion to the substrate. All these fundamental properties contribute to the high wear resistance of these coatings. However, carrying out full wear tests (e.g. ball-on-disc) is time-consuming, especially when it is necessary to select the proper feedstock material and carefully selected process parameters. The aim of the following researches was to statistically investigate the possibility of replacing long-term wear resistance tests with estimation of this performance on the basis of determining the fundamental mechanical properties of the coatings. Three types of coating materials were selected: WC-12Co, WC-10Co-4Cr and WC-20Cr3C2-7Ni, which were deposited on AZ31 magnesium alloy substrates from three different spray distances: 320, 360 and 400 mm. On the basis of the tests carried out and using cluster analysis techniques (the Ward and k-means methods), the relative similarity between the obtained coatings was determined. The applied methodology allowed to select from the analyzed cermet coatings such samples that were characterized by improved resistance to abrasive wear. The obtained results of the analyzes were also referred to the results of tests of resistance to abrasive wear. [ABSTRACT FROM AUTHOR]

The manuscript analyzes the impact of the HVOF (high-velocity oxygen fuel) coating spraying technology on a substrate made of a light and high-specific-strength magnesium casting alloy from the AZ31 series. Among others, the following were examined: the influence of the spraying distance of coatings using commercial cermet powders (WC–Co, WC–Co–Cr, and WC–Cr3C2–Ni) on their resistance to erosive wear. It is worth emphasizing the energy savings resulting from the possibility of spraying on the surfaces of existing machine parts to protect or regenerate them. Energy savings result from the possibility of recycling the substrate material (AZ31), as well as from extending the functionality of an existing element without the need to dispose of it and the energy-intensive production of a new component. Tests have shown that the best resistance to the destructive effects of erodent in the form of hard corundum particles is characterized by a WC–Co–Cr coating sprayed at a distance of 320 mm. [ABSTRACT FROM AUTHOR]

The study investigated the microstructures, residual stress and mechanical properties of double carbides cermet coatings manufactured on AZ31 magnesium alloy substrate by high-velocity oxy-fuel spraying (HVOF). The HVOF spraying was carried out using a JP 5000 TAFA spray system and the commercially available powder WC-20Cr3C2-7Ni was used to manufacture coatings. The variable process parameter was spray distance (320, 360 and 400 mm). The top surfaces of the manufactured coatings are typical for HVOF coatings and is relatively smooth. Nevertheless, some irregularities and voids are observed. All coatings exhibit dense structure with relatively low porosity level (below 3 vol. %) as well as a robust mechanical binding to the substrate which indicates high adhesion. More deep microstructure analysis carried out by TEM microscopy revealed two types of precipitation: WC hexagonal phase in the space group P-6m2 with irregular shape and size as well as Cr7C3 orthorhombic one, in the space group P nma with a rounded shape and regular size. Phase composition after the spraying process indicates the hexagonal WC phase as the main one. Additionally, two chromium carbides phases, Cr3C2 and Cr3C7 have been identified. A small peak of hexagonal W2C carbide was found as well. Analysis of residual stress showed that both components of linear stress for all deposited coatings have a compressive nature. The increasing spray distance caused an increase of the linear stress values, from – 72.7 up to – 131.5 MPa for δ11 and from – 57.2 up to – 112.2 MPa for δ22. Such values could suggest that thermal stress generated during coating deposition was also low. In the case of the shear stress the values are much lower than linear one, but with the same tendency, increasing with longer spray distance: 14.2, 21.0 and 32.2 MPa for 320, 360 and 400 mm, respectively. The obtained results of HIT show a slight influence on the spray distance. With the increasing spraying distance, the value of hardness decreasing: 12.96, 12.53 and 11.76 GPa according to SD equal to 320, 360 and 400 mm, respectively. Similar was in the case of the EIT values: 315, 309 and 304 GPa for 320, 360 and 400 mm, respectively. All values of the fracture toughness were in the range between 3.5 and 4.0 MPa m1/2, with little influece of the spray distance. [ABSTRACT FROM AUTHOR]

Magnesium alloys are very interesting engineering materials because of their high strength-to-density ratio. On the other hand, they are characterized by low hardness as well as low erosion resistance. Because of these reasons, their applications in the industry are very limited. The article presents the results of the high velocity oxy-fuel (HVOF) spraying of the hard cermet coatings onto AZ31 magnesium alloy substrate. Three feedstock powders were used in the process with composition (wt.%): WC-12Co, WC-10Co-4Cr and WC-20Cr3C2Ni. The spray distance (SD) was selected as a variable parameter with values equal to 320 and 400 mm. Observations carried out under a scanning electron microscope (SEM) revealed a typical HVOF-sprayed microstructure with a compact structure and low porosity (below 3 vol.%). The hardness of the manufactured coatings, ranging from 912 HV0.2 to 1328 HV0.2, what was significantly higher than the substrate. The solid particle erosion tests were carried out according to the ASTM G76-04 standard. Erosive experiments were done for 30°, 60° and 90° inclination angles of the nozzle using Al2O3 abrasive. Erosion tests confirm that cermets exhibit substantial erosion resistance better than the substrate. The highest erosion resistance was noted for WC-10Co-4Cr coatings. The erosion rate for cermet coatings was mostly below 0.9 mg/min, whereas for the AZ31 it was more than 1.5 mg/min. In the case of the average erosion value, it was between 12 and 22 times lower than for the substrate. Results analysis reveal that shorter spray distance decreases porosity, increases hardness, and finally supports erosion resistance of the cermets. [ABSTRACT FROM AUTHOR]

Cermet coatings provide protection against an aggressive operating environment of machine and device elements, such as corrosion, wear or high-temperature conditions. Currently, WC-based cermet coatings are frequently used in different industry branches. In this work, conventional WC-based powders (WC-Co and WC-Co-Cr) were sprayed with high velocity oxy fuel (HVOF) onto AZ31 magnesium alloy with different spray distances (320 and 400 mm). The research aimed to investigate the effect of the spray distance on the microstructure of the coatings, phase composition and electrochemical corrosion resistance. Results revealed that a higher spray distance results in greater porosity, 1.9% and 2.3% for 320 mm and 2.8% and 3.1% for 400 mm in the case of WC-Co and WC-Co-Cr coatings, respectively. Also, an impact on coatings microhardness was observed, c.a. 1300 HV0.3 for shorter spray distances, whereas for longer ones it was less than 1100 HV0.3. The corrosion resistance estimated in potentiodynamic polarization measurements was the best for WC-Co-Cr coating deposited from the shorter spray distance, corrosion current density was equal to 2:9 μA·cm-2 and polarization resistance was equal to 8424 Ω·cm2. [ABSTRACT FROM AUTHOR]

In the field of surface engineering, thermal spraying is very wide adopted in many branches of the industry. The main reasons of such situation are its flexibility as well as cost effectiveness. Among others, high velocity oxy fuel (HVOF) technique is dedicated for spraying hardmetal and cermet coatings, especially for wear- and corrosion resistance. Such type of coating could be a promising candidate as protective layer for magnesium alloys elements. These materials need a strong improvement in the corrosion protection as well as on the field of wear resistance in order to be widely used in the industry. In this work, different WC-based coatings, namely: (i) WC-Co, (ii) WC-Co-Cr and (iii) WC-Cr3 C2 -Ni manufactured by HVOF spraying, were investigated. The form of all feedstock materials was agglomerated and sintered powder. All coatings were sprayed with the same technological parameters, especially spray distance which was equal to 400 mm on the AZ91 magnesium alloy substrate. The main aim of the studies was to investigate the influence of the powder material on the corrosion resistance of obtained coatings. The manufactured coatings were examined in terms of its microstructure, using scanning electron microscope (SEM) and corrosion performance, which was assessed in the electrochemical corrosion investigations in 3.5% NaCl solution by Tafel method. The study showed that the corrosion resistance increasing in such order: AZ91 < WC-Cr3 C2 -Ni < WC-Co < WC-Co-Cr. It should be stressed that WC-Cr3 C2 -Ni coating exhibits very low corrosion performance, which could be effected by relatively high porosity (c.a. 3 vol.%) and because of that the more complex composition promotes creation of many corrosion cells. [ABSTRACT FROM AUTHOR]