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The enhanced optical properties of chemical vapor deposited (CVD) diamond have been pursued in academia and industry for many applications. However, the barrier of CVD technology limits the application field of diamond. Herein, the performance of CVD polycrystalline diamond thick films was improved by high-pressure and high-temperature (HPHT) treatment. The microstructures of CVD polycrystalline diamond films before and after HPHT treatments were thoroughly examined using optical microscope, UV–visible and infrared absorption, Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). It is found that the transparency of the CVD samples at 10 GPa increases dramatically with processing temperatures, from the original opacity to almost full optical transparency. Through spectroscopic and microstructural analyses, the modification mechanism of CVD polycrystalline diamond under HPHT conditions is proposed. The results show that the HPHT treatment can significantly enhance the optical properties of the starting CVD polycrystalline diamond films. [ABSTRACT FROM AUTHOR]

Polycrystalline diamond composites (PDC) have been used as a cutting material in a multitude of fields, from mechanical processing to oil and gas drilling. In this study, PDC samples were prepared under high pressure and high temperature (HPHT) conditions within a pressure interval of 5.5 to 10.5 GPa by sintering diamond powder (average grain size ~3 μm) with WC-10 wt% Co substrate. Samples obtained with varying sintering pressures and temperatures were characterized using Vickers hardness tester, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results revealed that: (1) within a specified material system, each distinct sintering pressure condition corresponds to a unique optimal sintering temperature that yields the pinnacle of consolidation efficiency in samples. This optimal temperature exhibits a linearly increasing trend with rising sintering pressure; (2) at the intersection where the sintering pressure attains its optimal match with temperature, an increase in pressure fosters more intimate contact and stronger inter-grain bonding among diamond particles, thus amplifying the D D bond formation effect, which consequently leads to a linear enhancement in the Vickers hardness values of PDC with increasing sintering pressure; (3) simultaneously, under conditions of optimal temperature matching, elevating the sintering pressure notably decreases the cobalt content within the polycrystalline diamond layer of PDC samples. These findings furnish crucial experimental evidence for optimizing PDC material properties, significantly contributing to the advancement of both theoretical exploration and technological innovation within the field, carrying profound academic value and practical guidance implications. [Display omitted] • At a given pressure, there is an optimal temperature for the peak sintering performance of PDC. • The optimal sintering temperature exhibits a linear increase with rising pressure. • Under optimal temperature conditions, increased pressure improves sample performance. [ABSTRACT FROM AUTHOR]

Pure diamond powder and mixed powder of diamond and sodium chloride were treated at high pressures (5 GPa and 10 GPa) and high temperatures (1000–2700 °C), and then the graphitization behavior of diamond powder under non-hydrostatic (pure diamond powder) and quasi-hydrostatic (mixed powder of diamond and sodium chloride) compression was quantitatively studied by analyzing the X-ray diffraction peaks' area of treated samples. It is found that the graphitization under non-hydrostatic compression is much more easily triggered by heating due to the heterogeneous stress distribution within the diamond grains. Under the quasi-hydrostatic compression of 5 GPa, the starting graphitization temperature is about 1600 °C, which is 300 °C higher than that under non-hydrostatic compression. Under a pressure of 10 GPa, graphitization could be observed after pure diamond powder was treated at 1400 °C, but no graphitization occurred in the mixed powder even after a high temperature treatment at 2300 °C. Optimizing the stress distribution among diamond grains by the mixing of grain-size can significantly reduce the graphitization at high temperature even under non-hydrostatic compression. [Display omitted] • Graphitization of diamond was quantitatively studied by x-ray diffraction peak areas. • The graphitization behavior of diamond is affected by stress state. • Higher temperature of graphitization is under quasi-hydrostatic compression of 5 GPa. • No graphitization occurs under quasi-hydrostatic compression of 10 GPa at 2300 °C. • The graphitization degree is greatly weakened by the mixing of grain-size. [ABSTRACT FROM AUTHOR]

The thickness of the silicon carbide layer around diamond grains in the reaction sintering process of diamond/SiC composites starting from diamond and silicon was investigated at 1100–1400 °C and 5 GPa using two different sample assemblies. Dense silicon carbide layers with thicknesses from 60 nm to 350 nm were observed using energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analysis. This demonstrated that the thickness of the silicon carbide layer increased with increasing sintering temperature and time. The surface morphology of diamond grains/single-crystal diamond wafers eroded by liquid silicon was analysed by field emission scanning electron microscopy (FESEM), and it was found that SiC growth was controlled by the diffusion of silicon and carbon atoms through the existing layer of SiC. [Display omitted] • The thickness of silicon carbide layer around diamond grains was measured at 1100–1400 °C and 5 GPa. Dense silicon carbide layers with thicknesses from 60 nm to 350 nm were observed using EDS and XRD analysis. • The thickness of silicon carbide layer increased with the increase of temperature and SiC growth was controlled by the diffusion of silicon and carbon atoms through the existing layer of SiC. • The quantitative data of the thickness of silicon carbide layer around diamond under high pressure is reported for the first time. [ABSTRACT FROM AUTHOR]

The stability and solubility of diamond in cobalt at a pressure of 5 GPa was investigated. At temperatures ranging from 1000 °C and 1350 °C, diamond can be dissolved in cobalt without graphitization. Meanwhile, the solubility of single diamond crystals in cobalt was measured at 5 GPa and temperatures of up to 1300 °C through weight analysis, and it was found to increase with the increase in temperature. The morphological changes of the diamond crystals after high-pressure and high-temperature (HPHT) treatments were observed using a scanning electron microscope. The resulting quantitative data and morphology analysis enabled the construction of a scheme for the morphological evolution of diamond crystals during their dissolution in cobalt at HPHT. The diamond formation process in cobalt solvent is discussed in relation to the dynamic equilibrium. Unlabelled Image • The solubility of diamond in cobalt was measured at 5.0 GPa and high temperatures up to 1300 °C. The temperatures region of diamond growth at 5 GPa is determined. • The solubility of diamond in cobalt increased with the increase of temperature and the diamond formation in metal solvents is a process of the dissolution and precipitation of carbon atoms. • The quantitative data of solubility of diamond in cobalt under high pressure is reported for the first time, which provides favorable evidence of solvent theory. [ABSTRACT FROM AUTHOR]