Dongkun Zhang

Bio: Dongkun Zhang is an academic researcher. The author has contributed to research in topics: Delamination & Coating. The author has an hindex of 4, co-authored 8 publications receiving 44 citations.

Evaluation of Delamination Strength of Thermally Sprayed Coating by Edge-Indentation Method.

Abstract: In order to evaluate the shear-delamination strength of thermally sprayed coatings, an “edge-indentation method” was newly proposed in the present study, and the method was applied to WC-Co coating deposited on a tool steel (JIS: SKD5) specimen by high-velocity flame spraying and Al2O3-TiO2 coating on a mild steel (JIS: SS400) specimen by plasma spraying. The delamination energy of coating, Ed, is defined by the following equation based on the indentation load P vs. displacement δ curve:Ed=∫δ00P·dδ-∫δ0-B10P'·dδ/S×2θ/2πWhere δ0 is the displacement at delamination, B1 is the coating thickness, S is the delamination area, 2θ is the delamination angle and P' is the indentation load only for substrate. The delamination energy Ed of WC-Co coating obtained by experiment reveals almost the same value irrespective of coating thickness B1 and relative indentation distance from edge x/B1. The Ed of Al2O3-TiO2 coating increases slightly with an increase in B1, and the Ed is smaller than that of WC-Co coating. The edge-indentation method is effective to evaluate the shear-delamination strength especially when the substrate is very hard.

Fracture Mechanics Analysis of Edge-Indentation Method for Evaluation of Delamination Strength of Coating.

Abstract: In order to discuss the delamination energy obtained by the edge-indentation method on a viewpoint of fracture mechanics, the crack initiation and propagation behavior were examined by the microscopic observation along interface under the indenter as well as the three dimensional finite-element stress analysis of the zone. The results show that the interfacial crack starts at a little distant position from the indentation center, and the triangular-shaped delamination of coating extends stably for a certain distance with increasing indentation load. When an apex of the triangular-shaped coating with partially delaminated interfacial crack is pushed by the side of indenter, the critical value of interfacial strain-energy release rate or the interfacial fracture toughness, 2γ12, is expressed by the following equation:2γ12=1/8π2k2B1Ee1cos2θ/tan2α(P0/x)2where B1 is coating thickness, Ee1 is elastic constant of coating, θ is a half of delamination angle, α is apex angle of indenter, P0 is indentation load at delamination, x is indentation distance from edge, and k is a constant. The interfacial fracture toughness, 2γ12, corresponds to the delamination energy, Ed, which has been defined experimentally from indentation load vs. displacement curve in the previous paper.

Analysis of Cracking and Delamination Processes of WC-Co Coating under Bending Load.

Abstract: n order to analyze the cracking and delamination processes of hard coating on ductile substrate under bending load, four-point bending tests were carried out for tool steel specimens with WC-Co cermet coating. The coating was divided by cracks repeatedly with increase in the bending load, and finally the delamination of coating occurred. The interval of cracks at the same nominal stress and those at the delamination ware large when the coating thickness was large. The stress distribution in the coating was analyzed by two-dimensional elastic-plastic finite element method. In addition, the stress distribution in the coating was calculated also by an elastic-plastic stress analysis, which introduced the tensile stress in the coating σx1(x) as follows:σx1(x)=β{1-cosh(αpx)/cosh(αpL/2)}where, L is the crack interval, x is distance from the center of divided coating, and αp and β are constants which contain thickness, material's constant of coating and substrate, and nominal bending stress. The cracking process calculated both by the finite element method and the analytical method agree relatively well with the experimental results.

Effect of Cyclic Surface Pressure on Delamination Strength of WC-Co Coating Sprayed by HP-HVOF.

Abstract: Substrate specimens of mild steel (JIS: SS400) and tool steel (JIS: SKD5) were coated with WC-Co cermet by High-Pressure High-Velocity Oxygen Fuel (HP-HVOF) spraying. Repeating pressure was applied on the surface of the specimens, and tensile tests and edge-indent tests were carried out to evaluate the tensile strength and the delamination strength of the coating.The tensile strength σC of WC-Co coating increases first and then decreases with increasing surface pressure cycles N when the maximum repeating pressure σmax is 240MPa, while σC increases with increasing N for σmax=480MPa. Both the interfacial fracture toughness Gc1/2 and the delamination strength Ed for SS400 specimens decreases with increasing N. On the other hand, Ed for SKD5 specimens does not change with N in each σmax. The SEM micrographs of cross section of coating show the existence of cracks parallel to the interface for SS400 specimens and they are developed with increasing pressure cycles, while the crack extension is small for SKD5 specimens.

Rolling contact fatigue of post-treated WC–NiCrBSi thermal spray coatings

Abstract: The aim of this experimental study was to comprehend the relative rolling contact fatigue (RCF) performance and failure modes of functional graded WC–NiCrBSi thermal spray coatings in the as-sprayed and post processed state, by means of Hot Isostatic Pressing (HIPing) and vacuum heating. Functional graded WC–NiCrBSi coatings were deposited by a JP5000 system. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, while vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. Rolling contact fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress and configuration, in full film elasto hydrodynamic lubrication. Results are discussed in terms of the relative RCF performance of the as-sprayed and post-treated coatings, and also surface and sub-surface examination of rolling elements using scanning electron microscope (SEM), light microscope and surface interferometry. Test results reveal that performance of the coating was dependant on the microstructural changes due to post-treatment. Coatings heat-treated at 1200 °C displayed superior performance in RCF testing over the as-sprayed coatings at all stress levels (2, 2.3, 2.7 GPa) with emphasis on RCF performance at lower stress load of 2 GPa, where no failure occurred. Improvement in RCF performance was attributed to the diffusion between the carbides and matrix resulting in improved strength. At higher levels of contact stress, failure was surface initiated, and was attributed to initiation and propagation of micro-cracks at the edge of rolling contact region which led to coating delamination.

Contact fatigue failure evaluation of post-treated WC–NiCrBSi functionally graded thermal spray coatings

Abstract: Functionally graded thermal spray coatings applied to industrial components, which are subjected to contact fatigue or repeated impact loading, can reduce components weight and internal stresses whilst improving the adhesive strength to combat surface and sub-surface crack propagation. However, defects within the coating microstructure, which cannot be removed by the functionally graded approach, can compromise components reliability in high stress tribological applications. Post-treatments such as vacuum heating and Hot Isostatic Pressing (HIPing) have been shown in scientific studies to improve the coating microstructure, however, the influence of post-treatment on thermal spray coatings in rolling/sliding contacts have been seldom reported. This paper reports the rolling contact fatigue (RCF) analysis of functionally graded WC–NiCrBSi coatings deposited by a JP5000 system and subjected to post-treatment. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, whilst vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. RCF tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration, in both full film and mixed elasto-hydrodynamic lubrication (EHL). Test results reveal that the performance of coatings was highly dependent on the changes within the coating microstructure. Coatings HIPed at 1200 °C displayed relatively improved RCF performance over the as-sprayed coatings at stress levels of 2 and 2.7 GPa in full film lubrication. Improvement in RCF performance was attributed to the densification and homogeneity within the coating microstructure.

Coatings and surface modification technologies: a finite element bibliography (1995–2005)

Abstract: This paper gives a bibliographical review of the finite element methods applied to the analysis and simulation of coatings, their mechanical and material properties from the theoretical and application points of view. The surface modification technologies in the context of coatings are also included. The added bibliography at the end of this paper contains 1032 references to papers and conference proceedings on the subject that were published in 1995–2005. The following topics are included: coating property simulations; surface modification process simulations and practical coating applications.

Direct measurement of interface strength between copper submicron-dot and silicon dioxide substrate

Abstract: We develop an experimental evaluation method of interface strength for ductile submicron-dots on a hard substrate without collapse of the dot. The validity is examined by a copper (Cu) submicron-dot on a silicon dioxide (SiO2) substrate with the rigid-layer of tungsten (W), which restrains the deformation and decreases the influence of complicated stress field due to the contact of tip. The diamond tip is dragged horizontally along the SiO2 surface and the load is applied to the side edge of the W layer at a constant displacement rate using a modified atomic force microscopy. Both the lateral and the vertical load and displacement are continuously monitored during the test. The lateral load, Fl, increases almost in proportion to the lateral displacement, δ l, and the Cu dot with the W layer is clearly separated from the SiO2 along the interface. The restraint by the W layer works well so that there are little damages in both the delaminated W/Cu dot and the substrate. The delamination lateral load, FlC, is successfully measured.

Finite element modelling and simulation of indentation testing: a bibliography (1990‐2002)

Abstract: This paper gives a bibliographical review of the finite element modelling and simulation of indentation testing from the theoretical as well as practical points of view. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1990 and 2002. At the end of this paper, 509 references are listed dealing with subjects such as, fundamental relations and modelling in indentation testing, identification of mechanical properties for specific materials, fracture mechanics problems in indentation, scaling relationship for indentation, indenter geometry and indentation testing.