J.M. Guilemany

Tribological study of NiCrBSi coating obtained by different processes

Thermal spraying offers a wide range of coatings with very different composition and properties. NiCrBSi is a Ni-base superalloy widely used to obtain high wear resistant coatings. This coating is usually heat treated after thermal spraying to improve their tribological properties. In this work a tribological comparison between NiCrBSi coatings obtained by spray&fuse and as-sprayed coatings obtained by Atmospheric Plasma Spraying (APS) and High Velocity Oxy Fuel (HVOF) spraying is carried out. Ball on Disk (BOD) tests are performed with a martensitic plain steel as counterface and wear parameters are calculated by means of Scanning White Light Interferometry (SWLI). Main wear mechanisms are investigated by the characterisation of the coating wear track and debris using Scanning Electron Microscopy (SEM). It is observed that different wear mechanisms take place in the coatings obtained by the diverse processes.

High-velocity oxyfuel Cr3C2-NiCr replacing hard chromium coatings

Comparative wear and corrosion properties of Cr3C2-NiCr (CC-TS) (a high-velocity oxyfuel [HVOF]) and hard chromium (HC) coatings obtained on a steel substrate have been studied. The structural characterization was done before and after measurements by optical microscopy, scanning electron microscopy, and scanning white light interferometry. Wear and corrosion properties were evaluated by ball on disk (ASTM G99-90), rubber wheel (ASTM G65-91), and electrochemical measurements of open circuit and polarization curves. The best corrosion and wear resistance was for the CC-TS obtained by HVOF. The open-circuit potential values measured for both samples after 18 h of immersion were: −0.240 and −0.550 V, respectively, for CC-TS and HC, versus Ag/AgCl,KClsat. Three orders of magnitude lower volume loss were found for CC-TS (HVOF) after friction tests compared with HC.

Effects of thickness coating on the electrochemical behaviour of thermal spray Cr3C2-NiCr coatings

Abstract Thermally sprayed HVOF coatings are increasingly being used in industrial applications where high wear and corrosion resistance are needed [1] , [2] . In this paper, electrochemical ac and dc experiments were used in order to obtain the corrosion resistance of coated steel with different numbers of Cr3C2–NiCr layers. This work has been performed in order to determine the role of coating thickness in the corrosion behaviour of a steel protected with cermet thermally sprayed coatings. It is known that a thicker layer protects better against corrosion when a metallic coating is evaluated. But cermet coatings, such as Cr3C2–NiCr, contain higher levels of porosity and residual stresses than metallic coatings, which really could influence the corrosion resistance of the deposited layer. Electrochemical measurements, such as Open-Circuit Potential (EOC), Polarisation Resistance (Rp) and Cyclic Voltammetry (CV), were performed in an aerated 3.4 NaCl media (%wt.). Electrochemical Impedance Measurements (EIS) were also done in order to obtain a mechanism that explains the corrosion process. Structural Characterisation was carried out by means of Optical and Scanning Electron Microscopes (OM, SEM) with an Energy Dispersive Spectrometry analyser (EDS). Results show that the corrosion resistance of the complete system is mainly influenced by the substrate behaviour. The application of a higher number of deposited layers did not substantially increase their anticorrosive properties. Stress generation during the spraying deposition process plays an important role in the behaviour of the coated steel against corrosion phenomena.

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 2

This is the second part of a review article that deals with an analysis of the influence of wetting at the substrate-coating interface and surface phenomena, substrate deformation, dynamics of splashing, splatsubstrate interaction and spraying at off-normal angles on droplet flattening, and the formation of splats in thermal spraying, which affects the coating quality. The results agree well with experimental data and improve understanding of the thermal spray processes to make them more effective.

Microstructural examination of HVOF chromium carbide coatings for high-temperature applications

Chromium carbide/nickel chromium coatings obtained by the high- velocity oxyfuel thermal spray proc-ess were characterized using conventional and high- resolution microscopy to identify the complex micro-structure that results from this thermal spraying technique. Thermal cycling and long isothermal treatment were studied, as were the adhesion properties of as- coated and thermally treated samples.

Effect of oxidation on droplet flattening and splat-substrate interaction in thermal spraying

The processes of oxidation that occur during particle inflight motion and during splat solidification in an oxygen-rich atmosphere were considered for the thermal spray process. The effect of oxidation on droplet flattening, splat-substrate mechanical and thermal interaction, splat morphology, and development of coating porosity and adhesion was studied. The influence of wetting and oxygen dissolution on flattening and splat-substrate adhesion was also investigated. The results from theoretical treatment agree with experimental observations.

Influence of surface roughness on the flattening of powder particles during thermal spraying

The time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of droplets during thermal spraying is investigated taking into account the surface roughness, splat solidification, and mass loss of the droplet liquid phase. Analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are found. Results agree well with the experimental data. They can be used to predict the splat flattening parameters.

Analysis of coating gas porosity development during thermal spraying

Abstract The formation of gas porosity during thermal spraying is investigated. The development of pores is studied, taking into account the parameters of spraying, powder particle material and substrate, droplet flattening and solidification kinetics, heat and mass transfer processes and pore interaction. The results obtained agree well with thermal spraying practice.

Oxidation of coatings in thermal spraying

Abstract Mechanisms of the coating oxidation during thermal spraying are discussed. They are associated with the in-flight oxidation of the powder particles and oxidation of the solidifying splat on the substrate surface. Influence of oxidation on the splat morphology, porosity and the splat–substrate adhesion is outlined. Theoretical results agree well with the observed tendencies of the development of oxidation and its effect on the coating formation.

Use of scanning white light interferometry in the characterization of wear mechanisms in thermal-sprayed coatings

Thermal spray coatings are widely used to reduce wear damage in certain engineering applications. There are several methods of measuring coating wear resistance. Among these, one of the easiest is the combination of the ball-on-disk test with interferometric measurements (noncontact profilometer). The main purpose of this article is to analyze the major wear mechanisms that occur in thermal-sprayed coatings tested under sliding conditions. This work shows how scanning white light interferometry can be easily used to study the wear mechanisms of some coatings, and allows abrasive, adhesive, and fatigue wear mechanisms to be distinguished. The main features of each of these mechanisms observed through the images of the interferometric microscope are reported.