ICMCTF2016 Session E2-2: Mechanical Properties and Adhesion

Nano-impact extends the reach of nanomechanical testing to much higher strain rates than are possible in standard nanoindentation. The repetitive high-strain contact allows the durability of coatings under intermittent highly loaded contact to be determined and the fatigue fracture resistance determined in the nano-impact test shows a very high level of correlation with coating performance in these types of contact situations - such as hard coatings for cutting tools or TBCs subject due to erosive impacts in jet engines - due to the closer simulation than in quasi-static tests.

A key step in optimisation of coating behaviour is to understand the relationship between a range of parameters that can easily be obtained in the repetitive nano-impact test and ultimate coating performance in demanding applications. In this presentation we have performed a statistical analysis of multiple repetitive impact tests on a range of advanced nitride coating systems and have identified the key parameters that can be used to assess durability and wear resistance.

Hard Ti- and Cr-based ceramic coatings have been proven to significantly enhance solid particle erosion and wear resistance compared to the ductile materials used in aerospace applications such as aircraft engine. More recently, monolithic ternary and quaternary TiN-based materials such as TiSiN, TiAlN and TiSiCN became commercially available, and they even surpass the binary coatings in terms of hardness and wear resistance. In these types of systems, mechanical properties (Young’s modulus - E, hardness - H, toughness) greatly depend on the additive content. In the present work, a finite element model has been developed to design a functionally graded coating system by investigating the influence of the E depth profile on the stress distribution induced by solid particle impact. The mechanical properties studied here were based on TiSiN that can be tailored through the silicon content control (E: 200-350GPa, H: 10-35GPa). Modeling results suggest that an optimal design (highest erosion resistance) is achieved by matching the properties at the interface to reduce localized shear stresses, while increasing the E value to its maximum to enhance load carrying capacity, and slightly decreasing it at the surface to hinder through-coating crack propagation. The model will be validated by experiment using PECVD TiSiN coatings.

Hard coating using physical vapor deposition (PVD) has been widely used in surface engineering over the past few decades, The well-studied TiN, TaN film and TiAlN film have been widely used in industry. Many studies were focus on their mechanical properties such as wear resistance, fracture toughness and hardness, etc. However, rare study on time and temperature mechanical properties measurement of them despite the wild variety use of them under high temperature conditions. With this investigation, TiN film was deposited on Si substrate by using a DC sputtering system. Various N2/Ar flow ratio with respect to 1/1, 1/5 and 1/9 were operated during PVD processes to obtain three different kinds of TiN films. Time and temperature measurement up to 200 °C on the mechanical properties of prepared TiN films were tested using bulge tests. The principle of bulge tests were to apply stress on TiN film and measured the difference in height by using laser light and position sensing detector system. With the pressure–deflection curve, the residual stress, time and temperature mechanical behavior of films were determined. Results show TiN_0.33 has stronger thermal stress resistance than TiN_0.22 and TiN_0.18 which implied that the increasing Nitrogen concertation during processes increase the thermal resistance of the TiN films. Surface morphology as well as microstructure of tested films was studied to discuss the processes effects on their mechanical properties.

Magnesium alloys attract the light-weight manufacture due to its high strength to weight ratio in the in the space industry. Despite their high strength to weight ratio, poor corrosion and wear performances limit their extensive usage. In this respect, improvement of their wear and corrosion resistance by coating processes. However, in many cases, the coating fails under mechanical loading due to the mechanical incompatibility of the coating and substrate. For this purpose, the MAO (micro arc oxidation) were used to deposit on AZ91 magnesium alloys . The surface topography, morphology, crystallographic structure and thickness of the coatings were determined using SEM, XRD. The hardness was measured using a microhardness tester. The adhesion and fatigue properties of the coatings were evaluated via a scratch test in two modes. A sliding-fatigue multimode operation was used that involved multi-pass scratching in the same track at different fractions of the critical load (unidirectional sliding), and a standard mode with progressive loading was also utilized. The results are showed that the coating parameters significantly affected the thickness, hardness and the adhesion properties of coatings.

The existence of residual stress gradient may affect reliability and performance on hard protective coatings. Measurement of stress gradient on thin films is challenging and critical mainly due to restricted thickness and unique microstructure. For polycrystalline films, X-Ray diffraction (XRD) is one the most common nondestructive techniques for measuring residual stress because of its excellent phase selective capability and extreme accuracy in determining interplanar distance. In addition, by adjusting the grazing incident angle of X-ray the variation of strain with thickness can be measured. The purpose of this research is to develop a simple and nondestructive method to assess the stress gradient in thin films using average X-ray strain (AXS) method [1] and layer-by-layer methods. The AXS method is a modified practice from cos²αsin²ψ XRD method [2], which is for enhancing the accuracy of X-ray stress measurement by increasing the diffraction volume at several azimuthal angles. By combining elastic constants from nanoindentation, the uncertainty of the measured stress, compared with that measured by optical curvature method, can be reduced to less than 10 % for TiN hard coatings ranging from 300 nm to a few μm [3]. In this research, we employed cos²αsin²ψ XRD method at different incident angles to measure the in-depth stress profiles of the coatings, where the AXS was applied to reduce the statistical error. Since the measured stress was an integrated magnitude from the surface to the penetration depth, a layer-by-layer method was adopted to resolve the real layer stress at different penetration depth. The layer-by-layer method was derived from layer removal method and X-ray absorption theory. TiN and ZrN coatings with thickness above 1.5 μm deposited on Si (100) wafer were selected as our model systems. The relation between thickness and stress gradient were explored. The data measured by the proposed method was compared with other techniques, and the advantages and disadvantages were discussed.

[1] A.N. Wang, C.P. Chuang, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 262 (2015) 40

[2] C.H. Ma, J.H. Huang, H. Chen, Thin Solid Films 418 (2002) 73.

[3] A.N. Wang, J.H. Huang, H.W. Hsiao, G.P. Yu, H. Chen, Surf. Coat. Technol. 280 (2015) 43