C. Godoy

Impact testing of duplex and non-duplex (Ti,Al)N and Cr–N PVD coatings

Abstract An impact test has been used to evaluate the impact resistance of duplex (i.e. pre-nitrided) and non-duplex PVD (Ti,Al)N and Cr–N coatings. The impact tests were carried out using a cyclic loading system developed at the University of Hull. Two different balls (tungsten carbide and hardened SAE 52100 steel) were used as impact bodies. After the test, samples were observed on the scanning electron microscope in order to investigate coating failure. Experimental results revealed that both duplex (Ti,Al)N and Cr–N coatings displayed better impact resistance than their non-duplex counterparts. None of the duplex coatings exhibited adhesive failures up to 5×10 4 impacts when both tungsten carbide and hardened steel balls were used. Conversely, all non-duplex coatings displayed adhesive failures at lower numbers of impact cycles. The nitrided case minimised substrate deformation under the hard PVD coatings so that fewer cohesive failures occurred in the duplex coatings. Moreover, the improvements of the duplex treatment could be more markedly noted when tungsten carbide balls were used, since higher stresses are expected to occur for this impact body. A mechanism involving material transfer from the ball to the coated surface was observed to occur only for the duplex (Ti,Al)N coating. A larger amount of transferred material took place for tests carried out with hardened steel balls, probably owing to their relatively lower hardness in comparison to that of the tungsten carbide balls. Such a mechanism was not observed to occur for the duplex Cr–N coating, which seemed to be the most suitable coating among all those tested, to be used in dynamic loading wear applications.

Toughness evaluation of hvof wc–co coatings using non-linear regression analysis

Abstract The models for predicting indentation toughness of materials are usually functions of both indenter and induced crack (i.e. Palmqvist or Half - Penny ) geometries. Palmqvist cracks have already been identified in sintered WC–Co cermets. In the present work, the indentation toughness of high velocity oxy-fuel thermally sprayed WC–12%Co coatings has been evaluated using a standard metallographic procedure. Two crack regimes were found to occur in such coatings, depending on the indentation load: at low indentation loads, Palmqvist cracks appeared whilst Half - Penny cracks were detected at high indentation loads. A statistical methodology was developed in order to determine the indentation toughness of WC–12%Co coatings, which were heat-treated at 1173 and 1463 K, from induced crack sizes measured at different loads. The non-linear regression analysis statistically confirmed the existence of two regimes for crack propagation and two distinct toughness values were recorded for the same material under different load conditions. The present results indicate that adequate post-heat treatments can lead to an increase in coating toughness.

Correlation between residual stresses and adhesion of plasma sprayed coatings: effects of a post-annealing treatment

In thermal spraying processes, residual stresses mainly arise from two sources: (i) shrinkage of the sprayed particles after solidification (primary cooling process) and (ii) differences between the coating (αc) and substrate (αs) thermal expansion coefficients (secondary cooling process). In the present work, residuals stresses in plasma sprayed NiCrAl coatings, deposited at different thicknesses onto two steel substrates (AISI 1020 steel, αc>αs and AISI 304 stainless steel, αc αs and αc αs; however, when αc<αs such agreement was not found. Coating adhesion was found to decrease with increasing coating thickness, indicating a correlation between adhesion and average residual stresses at the interface instead of adhesion and average residual stresses in the coating. After annealing treatments for 10 h up to 1073 K, the average tensile residual stresses in the coating and at the interface increased for the NiCrAl/AISI 1020 steel composite system whilst a decrease was recorded for the NiCrAl/AISI 304 stainless steel system. Such opposite behaviours could be attributed to differences between the αc and αs values of both systems. These results indicate that post-annealing treatments in plasma sprayed coatings should be carried out taking into account the differences between the coating and substrate thermal expansion coefficients, especially to improve coating/substrate adhesion.

An approach to elucidate the different response of PVD coatings in different tribological tests

Abstract In this paper, the different performances of duplex (i.e. with a prior plasma nitriding treatment) and non-duplex PVD coatings in micro-abrasive wear tests and impact tests are discussed, taking into account some key properties, such as nanohardness ( H ), elastic modulus ( E ) and the H / E ratio. The nanohardness and elastic modulus of the coatings were determined by nanoindentation and were characterised by means of several analytical methods, such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, Knoop hardness measurements, scratch tests and scanning electron microscopy (SEM). Similar H / E ratios were found for all coatings under investigation. Duplex (Ti,Al)N coatings exhibited the best wear resistance in micro-abrasive wear tests. The results obtained in such tests suggested the occurrence of abrasive wear by plastic deformation, with the hardest coatings displaying the lowest wear rates. The highest wear rates were recorded for duplex and non-duplex Cr–N coatings. Conversely, a duplex Cr–N coating exhibited the best performance in impact tests using a tungsten carbide ball. The duplex Cr–N coating displayed the lowest elastic modulus among all coatings tested, which can be attributed to the presence of a small amount of α-Cr metallic phase. This coated system also showed the smallest difference between the substrate and coating elastic moduli, indicating that low elasticity mismatch between the coating and substrate is desirable for achieving good performance in impact tests. The differing behaviours exhibited by duplex (Ti,Al)N and Cr–N coatings in different tribological tests reveal the importance of selecting the best coating for a given tribological application.

Micro-abrasive wear of PVD duplex and single-layered coatings

Abstract A micro-abrasive wear test has been used to evaluate the wear resistance of duplex coatings, consisting of plasma nitriding followed by PVD coating. Two duplex coatings were investigated: TiAlN and TiN. Single-layered TiAlN and TiN coatings were also tested to evaluate the effect of the duplex treatment on wear resistance. Prior to wear testing, coating properties were evaluated by hardness, scratch testing and surface profilometry. The best micro-abrasion resistance was shown by the duplex TiAlN coating, followed by single-layered TiAlN, duplex TiN and single-layered TiN coatings. The duplex treatment was effective in increasing the micro-abrasive wear resistance of its single-layered counterpart, since plasma nitriding prior to coating deposition increased the load support for the hard PVD coatings. The choice of the PVD coating also has a significant influence on the micro-abrasion resistance, with the more oxidation resistant TiAlN having a wear rate less than that of TiN. The results also indicate that coating debris acts as an additional source of abradant particles in the SiC slurry, leading to higher abrasive wear in the substrate. The harder the coating debris, the more severe the abrasive wear in the substrate is.

Surface Topographical Characterization of Concrete Protected by Coal Tar Using 3D Profilometry

The results obtained for the characterization of the surface texture of concrete protected by coal tar using a 3D-topographical approach are presented here. The concrete, which presents a low waterbinder ratio, high compressive strength, and high impermeability, has been subjected to an aggressive acidic environment by immersion in 2.5%H2SO4. The profilometry technique, which produces 3D-topographical images and amplitude parameters, is used to compare the coated and uncoated surface textures before and after chemical attack, to evaluate the degradation of concrete, and the minimization of such effect in the coated concrete. The degradation and the minimization of degradation can be confirmed by the variation in all the parameters studied. This research, based on studies of profilometric analyses, demonstrates the accuracy, precision and efficiency of this technique in analyzing the concrete surface, indicating that it can be broadly employed in concrete durability research. The methodology adopted demonstrates that the application of coal tar as a protective coating for concrete in an aggressive environment minimizes its surface degradation and increases its durability.

Influence of Surface Hardening Depth on the Cavitation Erosion Resistance of a Low Alloy Steel

In this paper, the influence of surface hardening depth promoted by plasma nitriding and Cr–Al–N coating deposition on the cavitation erosion resistance of a low alloy steel was investigated. Samples that were plasma nitrided for 2 and 4 h were produced and coated with 1 and 2 μm Cr–Al–N coatings deposited by plasma assisted physical vapor deposition. The characterization was carried out by X-ray diffraction (θ–2θ and glancing angle configurations), scanning electron microscopy, Rockwell C adhesion test, and three dimensional (3D) profilometry. Knoop microhardness tests were also performed. Cavitation erosion tests were carried out according to ASTM G32-03 Standard. The incubation period and cavitation erosion rate were determined. Coating deposition had a major influence on the incubation period, with thicker coatings resulting in longer times. Plasma nitriding treatment was more effective on reducing the average erosion rate. The plasma nitriding treatment and Cr–Al–N coating deposition in conjunction led to a decrease in both incubation period and erosion rate. The hardened systems presented mass loss up to 11 times lower than the nonhardened steel for the same time. It was concluded that the surface hardening was effective to improve the cavitation erosion resistance of a low alloy steel and the wear rate decreased with the increase of the hardening depth.