Elevated temperature repetitive micro-scratch testing of AlCrN, TiAlN and AlTiN PVD coatings

Abstract

In developing advanced wear-resistant coatings for tribologically extreme highly loaded applications such as high speed metal cutting a critical requirement is to investigate their behaviour at elevated temperature since the cutting process generates frictional heat which can raise the temperature in the cutting zone to 700-900°C or more. High temperature micro-tribological tests provide severe tests for coatings that can simulate high contact pressure sliding/abrasive contacts at elevated temperature. In this study ramped load micro-scratch tests and repetitive microscratch tests were performed at 25 and 500°C on commercial monolayer coatings (AlCrN, TiAlN and AlTiN) deposited on cemented carbide cutting tool inserts. AlCrN exhibited the highest critical load for film failure in front of the moving scratch probe at both temperatures but it was prone to an unloading failure behind the moving probe. Scanning electron microscopy showed significant chipping outside the scratch track which was more extensive for AlCrN at both room and elevated temperature. Chipping was more localised on TiAlN although this coating showed the lowest critical loads at both test temperatures. EDX analysis of scratch tracks after coating failure showed tribo-oxidation of the cemented carbide substrate. AlTiN showed improved scratch resistance at higher temperature. The von Mises, tensile and shear stresses acting on the coating and substrate sides of the interface were evaluated analytically to determine the main stresses acting on the interface. At 1 N there are high stresses near the coating-substrate interface. Repetitive scratch tests at this load can be considered as a sub-critical load micro-scale wear test which is more sensitive to adhesion differences than the ramped load scratch test. The analytical modelling showed that a dramatic improvement in the performance of AlTiN in the 1 N test at 500°C could be explained by the stress distribution in contact resulting in a change in yield location due to the high temperature mechanical properties. The increase in critical load with temperature on AlTiN and AlCrN is primarily a result of the changing stress distribution in the highly loaded sliding contact rather than an improvement in adhesion strength.

Effect of Nitrogen Doping and Temperature on Mechanical Durability of Silicon Carbide Thin Films

“…The multi-pass wear tests were used to mimic the very often in-service scenario of repetitive sub-critical stresses from single asperity contact 102 . They were performed as a sequence of alternating multiple scratch and topography passes, where the subcritical constant load of 300 mN that was chosen based on evaluation of ramped scratch test.…”

“…During the test each two on-load passes were followed by the topography pass (labeled as T). The test is often evaluated on the basis of the number of on-load passes without the traces of failure and can be therefore considered as a low-cycle wear test 102 . At least four ramped tests and two wear tests were carried out in all samples.…”

Effect of Interlayer Thickness on Nano-Multilayer Coating Performance during High Speed Dry Milling of H13 Tool Steel

“…The difference in wear resistance of the coating with a 500 nm interlayer that shows the highest initial penetration (consistent with its low near surface H/E and low Lc1 in the scratch test) but without failing after 15 cycles at 1.5 N can better explained in terms of the substrate roughness, Rt. It was previously observed [22] that a 50 nm Cr adhesive interlayer performed better than the 15 nm Furthermore, repetitive scratch tests performed at a subcritical load of 1.5 N (when all the studied coatings undergo crack initiation) are more sensitive to adhesion differences than the ramped load scratch test [41,42]. Hertzian analysis was applied on the ramped load test data for each coating and the position of maximum von Mises stress was found to occur at a depth of~1.5 µm below the free surface in the vicinity of the coating surface interface at a load of 1.5 N. Thus, observations from the repetitive scratch test at this load are more predominantly influenced by the coating layer than the deformation of the substrate.…”

“…Figure 8. (a-c) Microscopic images of the ramped load scratch test track showing the cohesive (a) and adhesive (c) failure within the studied coating (same failure mechanism for all the studied coatings). Furthermore, repetitive scratch tests performed at a subcritical load of 1.5 N (when all the studied coatings undergo crack initiation) are more sensitive to adhesion differences than the ramped load scratch test [41,42]. Hertzian analysis was applied on the ramped load test data for each coating and the position of maximum von Mises stress was found to occur at a depth of ~1.5 µm below the free surface in the vicinity of the coating surface interface at a load of 1.5 N. Thus, observations from the repetitive scratch test at this load are more predominantly influenced by the coating layer than the deformation of the substrate.…”

Elevated temperature micro-impact testing of TiAlSiN coatings produced by physical vapour deposition

“…Monolayered PVD (physical vapour deposition) ternary transition metal nitrides such as TiAlN and AlCrN have excellent wear resistance in metal cutting applications [1][2][3][4][5][6][7][8][9][10][11]. This is due to a combination of factors which act in synergy including improved oxidation resistance, thermal stability, age-hardening through spinodal decomposition and more protective tribofilm formation in comparison to binary nitrides such as TiN or CrN [12][13][14].…”