Yutao Pei

Laser cladding of ZrO2-(Ni alloy) composite coating

The microstructure of laser-clad 60 vol.% ZrO2 (partially stabilized with 2 mol% Y2O3) plus 40 vol.% Ni alloy composite coating on steel 1045 was investigated by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, energy-dispersive X-ray analysis and microhardness tests. The composite coating consists of a pure ZrO2 clad layer in the outer region and a bonding zone of Ni alloy adjacent to the substrate. The pure ceramic layer exhibits fine equiaxed ZrO2 grains in the outer zone and columnar ZrO2 dendrites in the inner zone, growing from the ceramic layer-bonding zone interface. This ceramic layer is composed of metastable t′-ZrO2 phase and a very small amount of m-ZrO2 phase and displays a microhardness of 1700 HV0.2. The high heating and cooling rate caused by laser cladding restrains the t → m phase transformation in the ZrO2 ceramic layer. Interdiffusion of alloy elements takes place in the bonding zone, in which the coexistence of ZrO2 particles, Ni-based solid solution and (Fe,Cr)23C6 particles in the interdendritic regions was found.

Tribological behaviour of laser-clad TiCp composite coating

The wear behaviour of laser-clad TiC-Ni alloy coatings was studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and pin-on-ring friction and wear tests. TiC particles can act as hard barriers to resist the scoring and plastic deformation of the matrix and then delay the occurrence of delamination without distinctly increasing the friction coefficient of the coatings. The degree of wear depends mainly on the extent of debonding and removal of TiC particles. With increasing normal load, mild scratching with fine grooves, oxidative wear with a thin layer of Ni2O3 film, delamination wear with serious plastic deformation and regular scale-like features were observed on the worn surfaces of coatings corresponding to various wear conditions. The softening and local melting of worn surface layers caused by friction heat produces a crushed crystalline or amorphous structure.

Microstructure of laser-clad SiC-(Ni alloy) composite coating

The laser cladding technique was used to produce Ni alloy coatings with different SiC particle (SiCp) contents on steel 1045. The complete dissolution of SiCp took place during laser melting and led to a microstructural evolution of the coatings associated with the SiCp content. M7X3 or M23X6-type carboborides and Ni-base solid solution are found as the main microstructural constituents of the clad layers, and the volume fraction of the carboborides increases with increasing SiCp content. When the SiCp content reaches 30 vol.%, a silicide of Ni3Si2 phase and a few spherical graphite precipitates are observed, and the γ matrix shows a modulated structure. The addition of SiCp can significantly enhance the microhardness and wear resistance of the coatings.

Laser Clad ZrO2-Y2O3 Ceramic/Ni-base Alloy Composite Coatings

Abstract A laser cladding technique was used to produce ZrO2ue5f8Y2O3 ceramic/ Ni-base alloy composite coatings on stainless steel 4Cr13. The microstructure and hardness of the composite coatings are analyzed by XRD, SEM, EPMA, TEM and microhardness testing techniques. A stratification is observed in the laser clad zone. The upper region of the clad is a pure ZrO2 ceramic layer, and the lower region is an excellent transition layer of Ni-base alloy. The ZrO2 ceramic layer exhibits equiaxed grains and columnar structure of t phase and a tiny network structure of m phase along the t boundaries. The transition layer of Ni-base alloy shows the multi-phase structure of primary γ-Ni dendrites and dendritic eutectic, at interdendritic regions, of (Fe, Cr)23C6 carbide plus γ-Ni phase.

Microstructure of bonding zones in laser-clad Ni-alloy-based composite coatings reinforced with various ceramic powders

Microstructure of the bonding zones (BZs) between laser-clad Ni-alloy-based composite coatings and steel substrates was studied by means of scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. Observations indicate that for pure Ni-alloy coating the laser parameters selected for good interface fusion have no effect on the microstructure of the BZ except for its thickness. However, the addition of ceramic particles (TiN, SiC, or ZrO2) to the Ni alloy varies the compositional or constitutional undercooling of the melt near the solid/liquid interface and consequently leads to the observed changes of microstructure of the BZs. For TiN/Ni-alloy coating the morphology of y-Ni solid solution in the BZ changes from dendritic to planar form with increas-ing scanning speed. A colony structure of eutectic is found in the BZ of SiC/Ni-alloy coating in which complete dissolution of SiC particles takes place during laser cladding. The immiscible melting of ZrO2 and Ni-alloy powders induces the stratification of ZrO2/Ni-alloy coating which consists of a pure ZrO2 layer in the upper region and a BZ composed mainly of y-Ni dendrites adjacent to the substrate. All the BZs studied in this investigation have good metallurgical characteristics between the coatings and the substrates.

Effect of tempering temperature on microstructure and sliding wear property of laser quenched 4Cr13 steel

4Cr13 martensite stainless steel was quenched by a CO2 laser and tempered for 2 h at different temperatures in the range 200 °C to 550 °C. The microstructure of treated layer was observed by SEM, XRD and TEM. Tempering leads to the decomposition of a large number of retained austenites in laser quenched surfaces 10 μm thick and the precipitation of chromium carbides which nucleate at phase interfaces, sub-boundaries and within laths. The worn surface and debris formed under various wear conditions were analyzed after a dry sliding wear test. With the increase of normal load or sliding speed, the transition of wear mechanism from mild oxidational wear to severe adhesive wear is observed. For laser quenched 4Cr13 steel, tempering at 350 °C for 2 h leads to the appearance of transition wear effects only at higher loads or faster speeds.