Deposition of hard and solid lubricant (TiN + MoSx) coating by closed-field magnetron sputtering; Julfikar Haider
Haider, Md. Julfikar (2005) Deposition of hard and solid lubricant (TiN + MoSx) coating by closed-field magnetron sputtering; Julfikar Haider. PhD thesis, Dublin City University.
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Vapor deposited thin coatings have served well as mechanically hard coatings in different practical applications e.g., cutting and forming tool industries, decorative industries, biomedical industries etc. Some of these coatings (i.e., TiN) have a high coefficient of friction and can cause abrasion and rapid wear of the opposing surface. In a closed-system, a hard coating constitutes a source of abrasive particles upon spallation within the mechanism, which can lead to a worse result than the uncoated surface. Increasing number of reports in literature has suggested the use of a hard solid lubricant based coating to avoid this problem.
A sputtering rig was made operative as a closed-field magnetron sputtering system by reestablishing different systems (magnetron, water cooling, vacuum, electrical etc.). A heavy-duty impact-abrasion wear tester was modified for applying lower load on the coated sample during abrasion by a spring-loaded mechanism. A single-axis and double-axis rotary substrate table system was designed, manufactured and successfully installed in the existing sputtering system. This substrate table facilitated the rig for the deposition of multicomponent and multilayer composite coating by using different target materials and reactive gases. A rotational control mechanism has been developed to rotate the substrate table at slower speed in front of activated targets and faster speed in front of inactivated targets within a single rotation instead of rotating at constant speed. This system increased the deposition rate thus saving time and materials.
A three dimensional Computational Fluid Dynamics (CFD) study has been carried out using FLOTRAN-CFD code of the ANSYS analysis package to predict the velocity, pressure and concentration distribution of the process gas species (argon and nitrogen) across the sputtering chamber, inlets and outlet. The numerical predictions provided useful understanding of the multiple species process gas distribution and their mixing behaviour at various gas flow rates to some extent.
Modelling of thermal stress developed during cooling of a typical coating-substrate system (TiNstainless steel) from deposition to room temperature was performed using the finite element code ANSYS. A parametric study was performed to analyse the effects of coating and substrate thickness, Young’s modulus, deposition temperature etc. on the thermal stress. Radial, tangential, axial and shear stress distribution through the thickness of the coating and substrate were examined. The interface of the coating and substrate was recognised as the critical location from the failure point of view. Ti interlayer between TiN coating and substrate has significant influence on the reduction of thermal stress at the interface.
The deposition of composite hard and solid lubricant coatings (TiN+MoSx) has been performed on a rotating substrate by closed-field magnetron sputtering using separate Ti and MoS2 targets in a nitrogen gas environment. The composition of the coating was varied by controlling target current, gas pressure, and target to substrate distance. Compositional and structural analysis indicated the presence coating species (Ti, N, Mo and S) and incorporation of Mo and S atoms in the TiN matrix. The change of process parameters during deposition was also reflected in the measurements. For instance with the increase of MoS2 target current, higher peak shifting and peak widening were observed in XRD study. The TiN+MoSx coating showed slightly lower hardness than pure TiN coating but higher hardness than MoSx coating. The hardness could not be directly correlated with the tribological properties. The adhesion test for all TiN+MoSx coatings showed a better adhesion compared to pure TiN and MoSx coatings. The TiN+MoSx coating also showed lower coefficients of friction than TiN in Pin-on-disk wear tests possibly due to the solid lubrication effect from MoS2 formed during sliding. This fact was also speculated based on the results of the optical investigation of the wear track.
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