Priit Kulu

Characterisation and modelling of erosion wear of powder composite materials and coatings

In erosion wear of composite materials at solid particle impact against the surface, plastic deformation, brittle fracture or combined mechanisms are dominating, depending on the properties of target material surface. Soft metal-matrix of composite materials requires a model of plastic deformation and the ceramic hard phase needs both of the models: the plastic deformation and the brittle fracture model of wear. In this paper, an attempt is made to model the erosion wear of composite materials, to characterise them using indentation method and to correlate calculated erosion rates with experimental results and material parameters.

High-temperature erosion of Fe-based coatings reinforced with cermet particles

High-velocity oxy-fuel sprayed, iron alloy-based powder coatings, reinforced with tungsten carbide–cobalt (WC–Co) and titanium carbide–nickel molybdenum (TiC–NiMo) cermet particles, are compared under high-temperature abrasive–erosive wear conditions. Both WC–Co and TiC–NiMo particles underwent fracture, as well as dissolution, during the spraying process, but in the case of WC–Co particles this process was remarkably less intensive. Under the low impact angle conditions, the WC–Co particle-reinforced coating exhibited 1.1 times lower wear than the TiC–NiMo particle-reinforced coating because of the larger amount of the reinforcement remaining. Under the normal impact angle conditions, the WC–Co particle-reinforced coating showed 1.2 times lower wear than the TiC–NiMo particle-reinforced coating because of the resulting larger size of the WC–Co reinforcement and a more ductile matrix.

Wear Behaviour of Recycled Hard Particle Reinforced NiCrBSi Hardfacings Deposited by Plasma Transferred Arc (PTA) Process

The main goal of present work was to study room temperature wear behaviour of advanced cermet particle reinforced hardfacings. For this purpose three different recycled powders: WC-Co, TiC-NiMo and Cr3C2-Ni were used as reinforcements for the commercially used NiCrBSi matrix alloy. Plasma transferred arc hardfacing process was used for deposition of the hardfacings. Manufactured hardfacings show only average wear resistance compared to WC/W2C reinforced NiCrBSi hardfacing with two times higher wear values. However, in combined impact/abrasive and erosive contacts, manufactured hard particle reinforced hardfacings have shown very promising results with high wear resistance. It was shown, that double structuring along with right material constituent and coating procedure selection may provide high wear resistance.

Influence of Laser Hardening to the Sliding Wear Resistance of the PVD (Al,Ti)N-G and nACo® Coatings

In the present article, the laser hardening of the carbon steel C45, previously coated by the physical vapour deposition (PVD) process, is studied. The (Al,Ti)N-G and nACo® (nc-AlxTi1-xN/α-Si3N4) coatings were applied. Nd:YAG laser with the laser beam power density of 1945 W/cm2 and scan speed of 300 mm/min was used for hardening process. Laser hardening lead to the formation of hardened layer under both coatings, consisting of austenite and ferrite. The approximate depth of the hardened layer and maximal microhardness was approximately 0.2 mm and 955 HV0.05 and 0.1 mm and 520 HV0.05 in the case of the (Al,Ti)N-G and the nACo® coating, respectively. After laser hardening the sliding wear of the (Al,Ti)N-G coating decreased by 1.25 times and of the nACo® coating by 1.05 times.

Abrasive Wear Resistance of Recycled Hardmetal Reinforced Thick Coating

The aim of the current study was to elaborate and compare abrasive wear resistance of thick coatings deposited with different hardfacing technologies. To produce metal matrix composite (MMC) coatings commercial iron and nickel based powders with recycled hardmetal content of 40 vol. % were studied. For deposition technologies plasma transferred arc (PTA) hardfacing, high-velocity oxy-fuel (HVOF) spraying were used. The microstructure of produced thick coatings was examined, including the distribution of hard phase and homogeneity of metal matrix. Micromechanical properties, including hardness and elastic modulus of features were measured by universal hardness measurements. Furthermore, behavior of coatings subjected to abrasive rubber-wheel wear (ARWW) and impact wear (AIW) tests were studied. Wear resistance of experimental PTA hardfacings at low velocity ARWW and AIW tests notably exceeds that of HVOF sprayed coatings. Wear mechanism dominating at abrasive wear in most cases is the removal of metal matrix due to lower hardness. Assignment of hardmetal waste as initial material can significantly decrease the cost of production, improve the mechanical characteristics of coatings and consequently increase their wear resistance. Results indicate, that the choice of matrix for the same reinforcements can also be as an important factor for combating abrasive wear. Fe-based thick coating, produced by PTA is more wear resistant compared to the Ni-based ones.

Influence of Surface Morphology on the Tribological Behavior of Diamond-Like Carbon Coating

Diamond-like carbon (DLC) films were prepared in-situ on top of hard coatings, namely, prior to the DLC deposition, CrN, TiCN and nanocomposite nACo (nc-AlTiN/(a-Si3N4) layers were deposited on WC-Co substrates. The Ra roughness of WC-Co substrates was 0.05 and 0.2 µm. Surface morphology of DLC coatings was investigated by means of profilometry and geometrical parameters Ra, Rsk, Rku and Rz were evaluated. Fretting tests were conducted with 3 mm in diameter alumina balls under 1 and 2 N load, at slip distance of 2 mm and frequency of 2 Hz. Statistical analysis shows significant correlation between Ra and Rz parameters corresponding to initial DLC surface and wear scar surfaces produced by fretting tests after 2400 and 4800 cycles, respectively. This finding indicates a relationship between friction (wear) and Ra and Rz parameters. Positive correlation between the coefficient of friction (COF) and kurtosis Rku indicates that surface flatness is an important factor for optimal friction of an alumina ball against DLC coating.

Comparison of Curvature and X-Ray Methods for Measuring of Residual Stresses in Hard PVD Coatings

Physical Vapour Deposition and PVD coatings are designed for several applications, from industrial to biomedical. Residual stresses, arising during coating deposition, have important effect on the coating’s service life as their influence to the mechanical and tribological properties. Our aim was to investigate the residual stresses in five different PVD coatings (TiN, TiCN, TiAlN, TiAlN, nc-(AlTi) N/α -Si3N4) (presence of the Ti as adhesion layer) by the layer growing curvature method and the X-ray diffraction techniques using a plate and a strip as the substrate. Residual stresses were compressive and very large (2.98 - 7.24) GPa in all coatings and comparable in TiN, TiAlN, TiAlN coatings in the case of both methods. The magnitude of residual stresses is influenced by intrinsic strain in the case of layer growth rather than by thermal stress.

Mathematical models for erosion and corrosion in power plants. A review of applicable modelling optimization techniques

Surface damage in power plants causes economic loss and waste of operational time in modifications or repair. The wear is caused mainly by solid particles and water droplets. Erosion and corrosion of protective coatings constitute a number of significant engineering difficulties. “Trial and error” type methodology to improve the materials design is rather expensive, inaccurate and time consuming. Mathematical modelling through optimization methods overcome these engineering complications/difficulties, and reduce the experimental/tribotesting period. In this paper we provide a brief review of the current classified erosion and/or corrosion models and, additionally, detailed modern optimization methods for precise modelling of given applications. A concise computational optimization example for an erosion model is presented with brief software details/approximations.

Investigation of Morphology Changes on Nanocrystalline Diamond Film Surfaces during Reciprocating Sliding against Si3N4 Balls

This paper investigates the morphological modifications of the nanocrystalline diamond (NCD) film surface under reciprocating sliding test conditions. The surface morphology was characterized by atomic force microscopy (AFM). We observed longitudinal grooves and transverse ripples which were formed during the sliding tests on the NCD film surface. The primary goal of the study was to understand the influence of frequency, sliding distance and load variations on the formation of ripple patterns on the wear scars surface. The morphological alteration from continuous to broken ripple shapes was observed. Our study suggests that the geometrical shape of ripples is affected by the formation of the periodic array of grooves.

Recycled hardmetal-based powder composite coatings: optimisation of composition, structure and properties

Hardfacing is among the most economical methods for surface treatment and service life and efficiency improvement of metal parts subjective to wear. At the same time use of low-cost recycling technologies in the production of powders is of current interest. Focus in this paper is on: a) the production of hardmetal/cermet powders as reinforcements for composite coatings; b) deposition technologies and optimisation of coating structure; c) the wear resistance of coatings at different abrasive wear conditions. The article describes the production and grindability of hardmetal (WC-Co) and cermet (Cr3C2-Ni, TiC-NiMo) powders and their size and shape. Results address the optimisation of the composition and structure of HVOF-sprayed and PTA-welded composite powder coatings based on the commercial spray powders and produced hardmetal/cermet powders. The abrasive wear resistance of coatings at different wear conditions (rubber wheel, erosion and impact wear tests) is analysed.