Keijiro Nakasa

Relation between delamination of thin flims and backward deviation of load–displacement curves under repeating nanoindentation

Abstract The delamination behavior of sputter coated TiN and AlN films have been examined from the load–displacement curves under repeated nanoindentation loading. When the penetration depth is either much larger or much smaller than the film thickness, the curve progresses in forward direction that represents the deformation of the substrate or film. On the other hand, a backward deviation is observed when the penetration depth is comparable with the film thickness. Observation of the indents by a scanning electron microscope (SEM) and an atomic force microscope (AFM) shows that cracking and extrusion of the film occur when the backward deviation appears. It suggests that the partially delaminated film releases the compressive residual stress and causes an upward bending due to the elongation of the delamintated film, which resists the penetration of the indenter in the consecutive cycle.

Analysis and simulation of cracking patterns in coating under biaxial tensile or thermal stress using analysis/FEM strain-accommodation method

Abstract The cracking patterns in coatings under biaxial tensile or thermal stress are analyzed by the “analysis/FEM strain-accommodation method” that combines the strain of the substrate with a coating obtained from thermo-elastic analysis with the strain of the substrate calculated by a finite element method. The simulation using this method is effective not only for expressing the cracking patterns observed in punch press tests of disk specimens with WC–Co cermet and Al 2 O 3 –TiO 2 ceramic coatings but also predicting the cracking process for the coating deposited on a part with a complex shape under thermal stress.

AN ANALYSIS OF ISLAND-DELAMINATION TYPE CRACKING PATTERN IN THE BRITTLE FILM COATED ON A DISK UNDER AXISYMMETRIC TENSION

Abstract In order to characterize the island-delamination type cracking patterns in the brittle film on a ductile substrate under uniform tension, an analysis based on energy consideration was carried out to introduce the relation among the size of island-crack, the tensile stress at the formation of island-crack, film and substrate thickness, surface energy of film, interfacial energy between film and substrate, elastic and plastic constants, Poisson ratio and yield strength of film and substrate. The bulge tests of ceramic and heat-resistant paint films on steel disks show that the size of island-crack increases and the pressure at the formation of island-crack decreases with increase in film thickness. The film thickness dependencies of island size and tensile stress at the formation of island-crack calculated from the analysis agree relatively well with the experimental results.

Effect of stress wave shape on the crack propagation velocity in cyclic delayed failure

Abstract Crack propagation velocity in delayed failure under superposed repeating load, ( d a/ d t) R , was compared with that under static load, ( d a/ d t) S Two peaks appear on the relation between decreasing rate of crack propagation velocity, 1-β = 1 − ( d a/ d t)) R /( d a/ d t) S and frequency, ⨍, both under sinusoidal and square load. By changing the ratio of holding time at maximum stress intensity factor to that at minimum stress intensity factor in square load, it was deduced that the existence of two peaks on the 1 − β vs f curve was caused by an asymmetric interaction between hydrogen atoms and cyclic moving of the position with triaxial tensile stress at crack tip. Moreover, the relation between 1 − β and f under the positive or negative saw tooth load could be well explained by the interaction model.

Backward deviation and depth recovery of load–displacement curves of amorphous SiC film under repeating nanoindentation

Abstract The deformation and delamination behaviors of sputtered amorphous SiC film have been studied from the load–displacement curves under repeated nanoindentation loading. When the penetration depth is either much larger or much smaller than the film thickness, the curve progresses in forward direction that represents the deformation of the substrate or film. On the other hand, when the penetration depth is comparable with the film thickness, large backward deviation and depth recovery around the minimum load are observed. Observation of the indents by an atomic force microscope proved that delamination starts from the portions beneath the indenter and progresses towards the shoulder of the indent. This behavior is strongly connected with the existing large residual compressive stress in the film. The difference in the delamination behaviors between SiC and TiN, AlN films is also discussed.

Evaluation of Interfacial Fracture Toughness of Thermal Barrier Coating under Heat Cycles

Thermal barrier coating (TBC) has been developed to protect gas t urbine components from oxidation. However, the delamination of coating sometimes occurs and the underly ing component is exposed to hot gas environment that may cause premature failure of t he components. In the present research, the method to estimate the delamination life of coating was proposed from the viewpoint of fracture mechanics. During the heat cycle tests, two types of delamination mode w ere observed. One was the partial delamination from the edge of the coating and another was the delamination by buckling in the center of coating. In order to express the delamination life of coating, the interfacial thermal strain energy release rate was introduced for both case s as a function of temperature difference at heat cycles, strain of specimen, thickness of coati ng, and materials and thermal constants.

Crack branching in delayed failure

Abstract The condition for crack branching in delayed failure was investigated on the basis of energy consideration, and the effect of diffusion behavior of hydrogen atoms on the crack branching was discussed. The branching angle predicted from the present theory well agreed with the experimental value. The stress intensity factor at crack branching, K IB , is dependent on the diffusion velocity of hydrogen atoms, i.e. branching appears to occur when the moving velocity of the position with tri-axial tensile stress reaches the upper limit value of diffusion velocity of hydrogen atoms. The time to crack growth initiation is actually independent of stress intensity factor when the crack branching occurs, probably because the diffusion velocity of hydrogen atom has a certain upper limit value.

The effect of repeating load on the crack growth initiation and crack propagation in delayed failure

Abstract The delayed failure test under repeating load was carried out with pre-cracked specimen. The incubation time and the crack propagation rate were correlated with the stress intensity factor K . The incubation time is decreased by the superposition of repeating load, as the range of stress intensity factor ΔK or the repeating frequency f increase. The reason can be explained by the promotion of corrosion reaction due to, e.g. the destruction of oxide film on the crack tip, which facilitates the invasion of hydrogen atoms into the material. The crack propagation rate da/dt is decreased by the superposition of repeating load, and there exist two valleys of crack propagation rate minima on the da/dt vs f and da/dt vs ΔK curves. One valley corresponds to the interaction between the cyclic movement of the region with tri-axial tensile stress and the hydrogen atoms diffused from crack tip, which disturbs the concentration of hydrogen atoms. Another seems te correspond to the generation of retained compressive stress which reduces the effective stress intensity at crack tip and supresses the invasion and diffusion of hydrogen atoms.

An analysis of crack propagation velocity in delayed failure under repeating load using an internal friction model

Abstract Frequency and temperature dependency of crack propagation velocity in delayed failure under superposed repeating load was analyzed using an internal friction model which assumes the interaction between hydrogen atoms and the cyclic moving of the position with tri-axial tensile stress at crack tip. The decrease of crack propagation velocity (da/dt)R by the superposition of repeating load, the appearance of minimum value in (da/dt)R at a certain frequency ƒ 0 , and the shift of ƒ 0 by the change of temperature, are well explained by the internal friction model. Another reason for the decrease of (da/dt)R by the superposition of repeating load appears to be the decrease of effective stress intensity at crack tip, though this cannot explain the appearance of minimum value of (da/dt)R.

Analysis of Cracking and Delamination of Sputtered Thin Films during Wear Process

Pure titanium substrate specimen was sputter-coated with SiC or CoCrNi-oxide thin film and the wear test was carried out by using a “ball-on-disk” type testing machine. The semi-circular parallel cracks appeared on the film surface with angles of 45 degree to the sliding direction of the SiC ball and the delamination of film quickly occurred after the cracking. With the aid of finite element methods, stress distribution before and after cracking in the film was obtained. The maximum tensile stress existing in the film at the back-contact edge of ball is the reason for the initiation of semi-circular parallel cracks. After the film is cracked, the tensile stress normal to interface as well as the shear stress along interface appear at crack tip. The combination of these two stresses is the main reason for the delamination after the film is cracked.