Correlations between microstructural parameters, micromechanical properties and wear resistance of plasma sprayed ceramic coatings

The micromechanical integrity of a ceramic plasma sprayed (PS) coating is determined by the size and distribution of the defects found in the coating, such as porosity, the inter-lamellar microcrack density, the intra-lamellar microcrack density as well as the lamellar, or splat, dimensions. In this work, several micromechanical tests were used to advance our understanding of the relationships between the different microstructural parameters found in PS ceramic coatings. The tests included depth sensing indentation, micro and macrohardness testing, and controlled scratch testing. Abrasive and erosive wear tests were performed on the same set of coatings, including plasma sprayed alumina and chromia, as well as sintered alumina as a reference material. The best correlations were found between the material hardness (H), the level of porosity (P) and the abrasive wear volume (W). Knoop hardness measurements provided the best correlation with wear data, followed by scratch hardness and Vickers hardness. An exponential function of the type W=k/Hⁿ was found, where k and n are constants. A similar function describes the correlation of wear volume with the elastic modulus of the coating. Fracture toughness could only be correlated with wear volume when combined with hardness in a function of the type W=k/H^0.5K_c^0.5. The incorporation into this function of a “microstructural factor” M=Pⁿ improves the correlation.     

Enhanced wear resistance of Ti-5Al-2Nb-1Ta orthopaedic alloy by nitrogen ion implantation

The tribological properties of nitrogen implanted Ti-5Al-2Nb-1Ta orthopaedic alloy was studied by performing lubricated pin on disc tests against ultra high molecular weight polyethylene pins. The results were interpreted on the basis of friction coefficient, wear volume loss and by characterising the wear debris to understand the wear mechanism. The results indicated a decrease in wear rate for implanted samples. Detailed investigations of the dose dependence on wear performance were carried out. The friction and wear data show a clear transition in wear modes between implanted and unimplanted alloys. The wear debris confirms the presence of titanium oxide and titanium oxynitride phases for untreated and nitrogen implanted alloy.     

Fretting wear behavior of nitrogen ion implanted titanium alloys in bovine serum lubrication

Nitrogen ion implantation was performed on biomedical titanium alloys by using of the PBII technology to improve the surface mechanical properties for the application of artificial joints. The titanium nitride phase was characterized with X-ray photoelectron spectroscopy (XPS). The nanohardness of the titanium alloys and implanted samples were measured by using of in-situ nano-mechanical testing system (TriboIndenter). Then, the fretting wear of nitrogen ion implanted titanium alloys was done on the universal multifunctional tester (UMT) with ball-on-flat fretting style in bovine serum lubrication. The fretting wear mechanism was investigated with scanning electron microscopy (SEM) and 3D surface profiler. The XPS analysis results indicate that nitrogen diffuses into the titanium alloy and forms a hard TiN layer on the Ti6Al4V alloys. The nanohardness increases from 6.40 to 7.7 GPa at the normal load of 2 mN, which reveals that nitrogen ion implantation is an effective way to enhance the surface hardness of Ti6Al4V. The coefficients of friction for Ti6Al4V alloy in bovine serum are obviously lower than that in dry friction, but the coefficients of friction for nitrogen ion implanted Ti6Al4V alloy in bovine serum are higher than that in dry friction. Fatigue wear controls the fretting failure mechanism of nitrogen ion implanted Ti6Al4V alloy fretting in bovine serum. The testing results in this paper prove that nitrogen ion implantation can effectively increase the fretting wear resistance for Ti6Al4V alloy in dry friction, and has a considerable improvement for Ti6Al4V alloy in bovine serum lubrication.     

Tribological behaviour of thermally sprayed silicon carbide coatings

SiC coatings have been successfully deposited using thermal spray detonation technique with a newly patented feedstock. Their tribological performance was compared to bulk SiC for dry and lubricated conditions (polyalphaolefin and 3.5 wt% NaCl solution). The lowest coefficient of friction (CoF=0.10) and wear-rate were detected with polyalphaolefin lubricant regardless of the test pair due to mixed fluid-film lubrication. Contradicting results were recorded under other test conditions. The coatings show low CoF of 0.20 in comparison to four times higher CoF of bulk SiC under dry sliding. Oppositely, SiC coatings in NaCl solution record five times higher CoF compared to bulk SiC CoF of 0.20. Such behaviour is associated with tribochemical reaction and tribo-corrosion mechanisms occurring in dry and NaCl sliding, respectively.     

Sliding wear evaluation of hot isostatically pressed (HIPed) thermal spray cermet coatings

The aim of this study was to investigate the potential of applying hot isostatic pressing (HIPing) as a post-treatment to thermally sprayed wear resistant cermet coatings. The relative performance of the as-sprayed and hot isostatically pressed functionally graded WC-NiCrBSi coatings was investigated in sliding wear conditions. Coatings were deposited using a high velocity oxy-fuel (HVOF)—JP-5000 system, and HIPed without any encapsulation at temperatures of 850 and 1200 °C. The influence of post-treatment temperature on the coating's wear resistance was thus investigated. Sliding wear tests were carried out using a high frequency reciprocating ball on plate rig using steel and ceramic balls under two different loads. Results are discussed in terms of microstructural investigations, phase transformations, mechanical properties, and residual stress investigations. The results indicated significant alteration of the coating microstructure, brought about by the coating post-treatment, particularly when carried out at the higher temperature of 1200 °C. As a consequence, developments in the coating mechanical properties took place that led to higher wear resistance of the HIPed coatings.     

Dry sliding wear response of some industrial powder coatings

This investigation analyses the sliding wear response of polyester and polyphthalamide powder coatings deposited by electrostatic spraying and ‘hot dipping’ fluidised bed. Tribological tests were conducted under dry conditions in a pin-on-disc arrangement, using a spherical counterpart. The experimental results showed that the deposition technology, the coating material and the thickness of the coating play key roles in determining the wear response of powder coatings. In particular, polyester coatings are deposited by a fluidised bed offer superior wear endurance, along with a low abrasion volume and a low wear rate. Conversely, polyphthalamide coatings are susceptible to faster wear by local cutting and plastic fatigue mechanisms.     

Tribological behavior of aluminum alloys surface layer implanted with nitrogen ions by plasma immersion ion implantation

Some 2014 and 2024 aluminum alloys were implanted with nitrogen ions (N⁺) by Plasma Immersion Ion Implantation (PIII), and dose range was from 2×10¹⁷ to 1×10¹⁸ N⁺ cm⁻². The microstructure of surface layer was studied by Transmission Electron Microscopy (TEM). The depth profile of the implanted layer was investigated by Auger Electron Spectrometry (AES). The wear test was carried on a pin-on-disk wear tester. The micro-morphology of wear was observed by Scanning Electron Microscopy (SEM). The results reveal that: after implanted with nitrogen ions, the friction coefficient of surface layer decreased, and the relative wear resistance increased with the increase of the nitrogen dose. The tribological mechanism was mainly adhesive, and the adhesive wear tended to become weaker gradually with the increase of nitrogen dose. The upper two effects were mainly attributed to the formation of hard AlN precipitation and supersaturated solid solution of nitrogen in the surface layer.     

Corrosion-wear behaviour of thermal sprayed nanostructured FeCu/WC-Co coatings

The corrosion and corrosion-wear behaviour of a thermal sprayed nanostructured FeCu/WC-Co coating were investigated in a Hank's solution and compared to stainless steel AISI 304 and nanostructured WC-Co coatings. Electrochemical noise and potentiodynamic polarization measurements were conducted along with an ex situ scanning electron microscopy to unfold the response of these materials under these corrosive and corrosion-wear test conditions. The multiphase structure of the FeCu/WC-Co coating induces a complex corrosion behaviour. Under corrosion-wear conditions, the nanostructured FeCu/WC-Co coating exhibits a depassivation/repassivation behaviour comparable to the behaviour of stainless steel AISI 304 and nanostructured WC-Co coatings.     

Low temperature investigations on frictional behaviour and wear resistance of solid lubricant coatings

The tribological characteristics of polymer-based solid lubricant coatings under frictional stressing in vacuum at 293, 120 or 77 K were studied. Vacuum-friction apparatus with cryogenic pumps and low-temperature tribometer designed at SR&DB of ILTP&E was used for sliding tests. It was found that the coefficient of friction is somewhat higher at low than at room temperatures. The effect of temperature decrease on the wear life of solid lubricant coatings is ambiguous and determined by the direction of changes in physical and mechanical characteristics of a solid lubricant coating under cooling and by the rate of the process of a binder tribodestruction.     

Fretting-induced friction and wear in large flat-on-flat contact with quenched and tempered steel

Fretting may cause severe surface damage and lead to unexpected fatigue failure. Our test apparatus was designed based on reciprocating, large, annular flat-on-flat contact without any edge effects in the direction of the fretting movement. Fretting wear tests were run with quenched and tempered steel with different normal pressures and sliding amplitudes under gross sliding conditions. The development of the friction coefficient and total wear mass depended mostly on the accumulated sliding distance. Initially, friction and wear were highly adhesive but gradually changed to abrasive due to third body accumulation in the interface.     

Wear and oxidation behaviour of shrouded plasma sprayed fly ash coatings

In a wide variety of applications, mechanical components are subjected to severe conditions, such as high load, speed or temperature and hostile chemical environment. Thus, their surface modification is necessary in order to protect them against various types of degradation. Thermal spraying has emerged as an important tool of increasingly sophisticated surface engineering technology. Plasma spraying is a rather simple process from a practical point of view. In the present investigation, the fly ash coating has been obtained by shrouded plasma spray process on carbon steel. The coating was characterized with relative to important behavioural parameters. Wear, oxidation and salt corrosion behaviour have also been evaluated. The coating has found to posses much higher hardness values than base steel and its porosity is slightly higher than the range of porosity for plasma spray coatings. The coating was effective to increase the oxidation and salt corrosion resistance of the given carbon steel. However, the wear resistance of fly ash coated steel was observed to be lesser than the bare steel probably due to coarse grain size.     

The erosion and abrasion characteristics of alumina coatings plasma sprayed under different spraying conditions

A series of plasma sprayed alumina coatings was evaluated regarding their erosion and abrasion characteristics. The coatings were deposited under different spraying conditions, using a commercial axial injection plasma spray system, and with powders of different grit sizes and crystallinity. A sintered bulk alumina and a conventionally sprayed coating, produced by a radial injection air plasma spray technique, were tested as reference materials. To evaluate the importance of energy input the coatings were produced using two different torch nozzle sizes and gas mixtures with a varied amount of hydrogen. The erosion and abrasion results indicate that hydrogen concentration, nozzle size and precursor powder type and size influence the tribological characteristics of the coatings. The wear resistance of the coatings seems to benefit from an increase in hydrogen concentration or torch nozzle size. The effect of precursor powder size on the wear resistance was more complex but indicates that sapphire powders of medium precursor sizes are advantageous to ordinary plasma spraying powders of alumina.     

Nanoscratch and nanowear testing of TiN coatings on M42 steel

Abstract

Constant load, progressive load and multipass nanoscratch (nanowear) tests were carried out on 500 and 1500 nm TiN coatings on M42 steel chosen as model systems. The influences of film thickness, coating roughness, scratch direction relative to the grinding grooves on the critical load in the progressive load test and number of cycles to failure in the wear test have been determined. Progress towards the development of a suitable methodology for determining the scratch hardness from nanoscratch tests is discussed.     

Characterization of mar/scratch resistance of coatings with a Nano-indenter and a scanning probe microscope

Mar/scratch resistance characterization techniques are presented in this paper, in which a Nano-indenter and a scanning probe microscope (SPM) are used to measure the micro mar resistance (MMR), different responses of coatings to the marring stress, and critical forces for rough trough, cracking, delamination, and chipping, quantitatively. It provides a full spectrum of the mar/scratch resistance behavior of the tested samples. To comprehensively evaluate a coating/material in a specified application, introduction of a quantitative index is very useful. The concept of the index and the procedure to calculate it are described in the paper, too. In addition, to meet the variety of coatings/materials properties and the requirements in their applications, some complementary test methods are discussed.     

Evaluation of wear and tribological properties of coatings rubbing against copper

A new crossed-cylinders tribo-tester is proposed. This tribo-tester can decrease the tendency of the chatter vibration. The tribological properties of coatings against copper is evaluated with this tribo-tester. The wear rate of TiN, TiC and TiCN rubbing against copper is higher than the substrate high speed tool steel: SKH51 (JIS). The catalytic action of copper for oxidation of Ti-based coatings is a main reason of this high wear rate of TiN, TiC and TiCN rubbing against copper. The wear rate of CrN rubbing against copper is in a very low level because CrN shows the excellent oxidation resistance and Cr₂O₃ film formation decreases the wear loss of CrN coating.     

Quantification of wear,strain and heat energy consumption rates for sliding steel ball against diamond-like carbon coatings

Abstract

In this work, we evaluated the different energy consumption rates associated with the total frictional energy for a ball sliding on a flat surface. The energy generated by the sliding two bodies in contact is dissipated into the materials in various forms. The wear consumption energy for a steel ball against a diamond-like carbon surface was evaluated by the wear coefficient of the wear volume–energy input equation. The strain energy generated in the steel ball as a result of being made to slide under a certain load was calculated using the Hertzian theory. The chemical reaction energy was estimated based on iron oxidation. Finally, the frictional energy dissipated as heat was obtained by subtracting the wear and the strain energies from the total frictional energy.     

Evaluation of erosive wear resistance of TiN coatings by a slurry jet impact test

In this paper, it is proposed to use a new type of solid particle impact test (slurry jet) to swiftly evaluate wear properties of thin, single layered or multilayered coatings. By the slurry jet, 1.2 μm alumina particles were impacted at high velocity perpendicular to thin PVD coatings of TiN deposited on high speed steel substrate materials under various substrate temperatures. Since the coatings have a much higher wear resistance than the substrate material, the wear rate increases significantly to the higher level of the HSS material when the coatings are penetrated. This is utilized in the quantification of the assessment of coating wear. A ranking of wear resistance and correlations to the coating surface hardness measured by nano-indentation tests, and coating morphology and structures are given and discussed. The TiN deposited under the highest substrate temperature proved to have the highest wear resistance although it had a relatively low hardness. The wear rate of the TiN coatings varies with the orientation of grains, that is, the {1 1 1} orientation that dominates for the high temperature deposition shows a higher wear resistance than the {1 0 0} orientation, which corresponds with the cleavage fracture behavior. Thus, it can be recommended as a screening test when evaluating coatings and coated materials.     

Wear resistance and anti-sticking properties of duplex treated forming tool steel

The aim of this work was to investigate the potential of using hard physical vapour deposition (PVD) coatings on forming tools, as well as to determine the influence of plasma nitriding on the load-carrying capacity and wear resistance of coated tool surfaces. A load-scanning test rig was used for evaluation, where duplex treated cold work tool steel samples were loaded against soft austenitic stainless steel and hardened ball bearing steel, respectively. Four different coatings (TiN, TiB₂, TaC and DLC) and two substrate treatments (hardening and plasma nitriding in two different gas mixtures) were included.Plasma nitriding alone significantly improved the friction, wear, and anti-sticking properties of the tool steel. PVD coating, and especially PVD coating of nitrided tool steel further improved the performance. Therefore, from the point of view of tool life as well as work peace surface quality, the DLC coating with its excellent anti-sticking properties and sufficiently good wear resistance represent the best solution for forming tool applications of austenitic stainless steel.     

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.