David Rats

Characterization of diamond films deposited on titanium and its alloys

Abstract Titanium and its alloys have important applications for example in aerospace or as bioimplants. Some of these applications would be improved by diamond coatings. However the large thermal expansion mismatch between diamond and titanium or its alloys creates high residual stresses, up to about 7 GPa at 800 °C, which represent an important drawback. In this study, polycrystalline diamond films were deposited on pure titanium and Ti-6Al-4V in a classical tubular microwave plasma reactor from C-H(-O)-containing gas mixtures, at a temperature in the range 600–900 °C. Raman spectroscopy provided information about the diamond grain stress, which is obviously related to the deposition temperature. X-ray diffraction indicates the presence of titanium carbide or oxycarbide. Some other characterizations by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) are reported. It is shown that XPS coupled to argon ionic etching allows us to study the first steps of the deposition process. The structure and the chemical composition at the interface of a thicker deposit are obtained by TEM and EELS.

Crystalline quality and residual stresses in diamond layers by Raman and x‐ray diffraction analyses

A major problem for diamond coating applications is that diamond films tend to exhibit poor adherence on many substrates and typically disbond at thicknesses of the order of a few micrometers principally because of residual stresses. Residual stresses are composed of thermal mismatch stresses and intrinsic tensile film stresses induced during film growth. Diamond films were deposited in a classical tubular microwave plasma reactor from hydrocarbon–hydrogen–oxygen gas mixtures. The stress level was investigated as a function of the gaseous composition (especially oxygen concentration) and deposition temperature (700–900 °C). Thermal stress was directly calculated from Hooke’s law using a biaxial Young’s modulus value of 1230 GPa for polycrystalline diamond. Total diamond film stress was directly determined by the radius of curvature method and by x‐ray diffraction using the sin2 ψ method. The microdistorsions and the size of the coherently diffracting domains have been determined from the broadening of the...

In-depth structural X-ray investigation of PECVD grown diamond films on titanium alloys

Diamond coatings have been deposited in a microwave plasma from CH4–H2 and CO–H2 mixtures, with the aim of improving the wear resistance of titanium alloys. In-depth distribution profiles of crystalline phases formed during the diamond films growth are measured here using Grazing Incidence X-ray Diffraction (GIXD), in correlation with Scanning Electron Microscopy (SEM) coupled with EDX analysis. The GIXD method is particularly well adapted to study surface modifications or depositions of polycrystalline materials. Especially in this case, the formation of different intermediate carbonaceous layers are evidenced in terms of microstructure, from the diamond films to the titanium alloy substrate. In addition, crystalline parameters, microstrain rates and coherent domain size evolutions of different α and β-Ti alloy solid solutions, titanium carbide and diamond phases have been studied with respect to growth kinetics and deposition temperature, at 600°C and 850°C.

Tribological study of diamond coatings on titanium alloys

Abstract This paper presents the tribological characteristics of PACVD diamond coatings on titanium alloys. The coating morphology and quality have been investigated by SEM and Raman spectroscopy, and the surface roughness has been determined by AFM measurements. Friction and wear characteristics have been studied with a pin-on-disk tribometer as a function of the applied load, the sliding speed and the counterface material. For diamond-on-diamond contact, the dynamic friction coefficient stabilizes at a value of 0.05–0.08 after the surface polishing of diamond crystal tips. For non-diamond coated pins, a surface running in time at low sliding speed is necessary to avoid a coating spalling on the disk. After this step, the sliding speed can be increased to 1 m s−1 with a friction coefficient equal to or lower than 0.1. The wear of the diamond-coated disk generally cannot be measured.

HREM and EXELFS investigation of local structure in thin CVD diamond films

Abstract Diamond coatings prepared by microwave plasma-assisted chemical vapour deposition (CVD) on silicon substrates have been studied by two complementary transmission electron microscopy techniques, high resolution electron microscopy (HREM) and extended energy loss fine structure spectroscopy (EXELFS). HREM observations allowed a wide range of growth defects, mainly based on coherent twinning on {111} planes, to be identified. But 5-fold twin centres were very rarely found, whereas the images often revealed extended areas exhibiting the perfect diamond structure, both observations indicating a film quality higher than usual for synthetic diamonds. Moreover, the radial distribution functions (RDFs) derived from EXELFS spectra confirmed that the material was quite well-ordered. Typical examples are given, of a 〈110〉 image exhibiting different twin and grain boundaries of types Σ = 3, 9, 19, 27, 33 and 81, as well as one 5-fold twin centre, and of an extended perfect structure area viewed along the 〈111〉 direction. A particularly interesting defect configuration, consisting of three neighbouring dissociated dislocations, is also analysed: one of these, a pure extended-screw dislocation, allowed us to determine the intrinsic stacking-fault energy in diamond, yielding a value of 276±3 mJ m −2 . This value, although lower than the theoretical predictions, agrees very well with mean values previously determined from weak-beam experiments.

Diamond coating of titanium alloys at tempering temperature

Abstract Polycrystalline diamond films have been deposited on pure titanium and Ti-6Al-4V in a microwave plasma reactor from CH 4 -H 2 and CO-H 2 gas mixtures, at a temperature in the range 600–900 °C. X-ray diffraction, scanning electron microscopy and Raman spectroscopy showed that 1–10 μm diamond layers could be produced at 600 °C with a fairly good crystallinity and residual stresses limited to about 5 GPa. The intrinsic properties of the heat-treated titanium alloys are maintained at this tempering temperature and no spalling occurred when an appropriate pretreatment was carried out. Therefore we expect that diamond-coated titanium alloys could be used for general engineering applications where a low friction coefficient, low wear rate and load-bearing capacity are all necessary.

Pressure influence on the diamond deposition domain from various CHO(-Ar)-containing gaseous mixtures

Abstract Plasma volume can be increased through the addition of rare gas or by using low pressures. The influence of these deposition conditions on the diamond growth from CHO-(Ar)-containing gas mixtures was studied in a classical microwave plasma reactor. The morphology, grain size, growth rate and uniformity of diamond films were investigated by SEM and their quality was deduced by micro-Raman and SIMS studies. Their variations with different gaseous compositions and pressures are reported and discussed. It is especially shown that low pressures decrease the film quality and the diamond deposition domain. However, pressures in the range 2–5 Torr could be employed for mechanical applications, for example.