SEM images of a single and b multi-scratched thin film multilayer structures made using a normal load of 2 mN [ 26 ]. SEM images of the thin film multilayer structure cross sectional surfaces made by a single scratch, b 2 c 4 and d 6 multi-scratches under a normal load of 2 mN the scratching speed and pitch distance were kept constant [ 26 ]. Maximum shear stresses at different interfaces during scratching with critical normal loads for inducing delamination at corresponding interfaces [ 26 ].
Thin Films and Coatings: Toughening and Toughness Characterization
Although being widely used in the understanding of deformation and removal mechanisms involved in abrasive machining, nanoscratching has its limitations in simulating the practical machining process. This is because the lateral speed of the indenter is much lower than the working speed of abrasive grits during lapping, polishing or grinding. Also, nanoscratching does not take the thermal effect into account. In abrasive machining, temperature rise at the contacting area between abrasives and workpiece could be significant, which might affect interfacial delamination.
AFM left column , SEM micrographs central column and magnified images right column of the cross-sectional surfaces of the multilayer shown in Fig. Roughness values of different layers plotted as a function of the residual scratching depth measured from nanoscratching and lapping [ 26 ]. The grinding of the cross-sectional surface of a thin film multilayer shows more complicated characteristics of deformation and failure. Sumitomo et al. The grinding tests were carefully conditioned in order to achieve ductile mode removal.
Nevertheless, their results revealed that fractures were still observed on the thin film layers even though the measured grinding streak depth was smaller than the ductile-brittle transition cutting depth calculated by the Bifano model [ 90 ], which was likely because the crack initiated at interfaces propagated into the neighboring layers. Apparently, the material removal mechanisms of thin film structures involved in abrasive machining were different from bulk materials.
Delamination at interfaces was found to be more easily to occur than fracture on the brittle layers for this multilayer structure. Either reducing the grit size or feedrate during grinding would improve the interfacial quality. Obviously, the improvements were achieved through decreasing the aggression level of machining, but the trade-off was the sacrifice of productivity.
The previous studies on the deformation and interfacial failure mechanism of thin film multilayer structures demonstrated some common removal characteristics in nanoscratching and abrasive machining. However, thermal effect is expected to have much more significant impact in abrasive machining than that in nanoscratching. In the future work, nanoscratching should be performed at elevated temperatures, so the role of thermal effect in the interfacial delamination of thin film multilayer structures can be clarified. Through thickness cracks were observed inside thin film layers after nanoscratching and abrasive machining.
Such cracks are expected to alter the material removal mechanism during machining, as stress distribution would be varied. However, in most of the modelling works published, the effect of stress status at the interface on crack formation and propagation was not taken into consideration due to the limitation of modelling.
It is documented that the use of cohesive zone models can be an effective method to simulate material fracture [ 94 , 95 ]. The application of such models would not only give the interpretation of crack initiation, but provide detailed insight into the evolution of stress during interfacial debonding. The deformation characteristics and failure modes in thin film bilayer and multilayer structures induced by nanoindentation and nanoscratch were systematically reviewed. Under such mechanical loads, three typical failure modes could be identified, namely interfacial delamination, through-thickness cracking and interfacial failure induced by substrate cracking.
Among the three failure modes, interfacial delamination is the failure mode occurred most frequently as interfaces between thin films are likely the weakest link in most of the bilayer and multilayer structures.
Nanoscratch appears a viable tool to understand the deformation characteristics and removal mechanism involved in the abrasive machining of thin film multilayer structures, whose results agree well with those obtained from lapping and grinding. However, the requirement on ductile mode machining is somehow different from that for bulk materials as interfacial failure is the most likely failure mode in abrasive machining and the conditions for eliminating interfacial failure is more stringent than those for avoiding brittle damages in bulk materials.
Both shear and tensile stresses generated in abrasive machining are attributed to the cause of interfacial failure in thin film multilayers, but shear stress seems playing a more significant role. Skip to main content Skip to sections. Advertisement Hide. Download PDF. Deformation, failure and removal mechanisms of thin film structures in abrasive machining. Open Access. First Online: 18 February In the past few decades, the mechanisms that control the structure failure of thin film systems have attracted great research interests.
Many researches have been conducted to determine the mechanical behaviors of thin film structures under mechanical loading. Evans et al. It is obvious that three types of failure modes are commonly seen in the thin film structure: i films subject to residual compression are susceptible to buckling and spalling, where delamination would initiate at the interface; ii brittle films under residual tensile stress are liable to form cracks inside the layer; iii for ductile films subject to residual tension, film decohesion is usually triggered by substrate cracking that propagates to interfaces.
The schematic illustrations of the three types of failure are shown in Fig. Open image in new window. The failure mechanisms in a hard coating on a soft substrate were explained by Chen and Bull [ 40 ]. In the case of a thin film well bonded to a substrate shown in Fig. Tension at the interface was generated when the tip was withdrawn from the substrate. Once the interfacial tensile stress exceeded the threshold value, the hard coating would be torn apart from the substrate, as can be seen in Fig. If the bonding between a hard film and a substrate is poor, the mechanism of the interfacial failure is somehow different.
In this case, with the indenter penetrated deeper, the film deflected into the plastically deformed impression on the substrate. At this point, high bending stress was generated at the contact edges, driving the thin film detach from the substrate. When the interfacial crack length reached its critical buckling length, double buckling could be formed during indentation [ 42 ].
After the indenter withdrew, the interfacial cracks would propagate toward the middle of the indent as the thin film was no longer under constraint. Therefore, the double buckling failure outside the contact area could be developed into single buckling upon tip removal [ 40 ].
It is apparent that for a bilayer structure, in which the thin film possesses higher yield stress, the softer substrate is likely to yield first during indenting and deform plastically, while elastic deformation is still predominant in thin films [ 40 , 43 , 44 ]. As a consequence, the mismatch in elastic recoveries during unloading would build up interfacial stresses and hence significantly influence the deformation characteristics.
Lu et al. Both pop-in and pop-out events were observed from the P—h curve when the normal load applied was sufficiently high, as shown in Fig. The pop-in event was attributed to the plane slip occurred in the GaAs substrate, and the appearance of the pop-out was identified as a sign of interfacial delamination during tip withdrawal. Interfacial failure could occur at both loading and unloading stages with different delamination types [ 27 , 45 , 46 ].
Different crack initiation modes were observed in the model and analyzed using the linear elastic fracture mechanics LEFM , as shown in Fig. A tangential or sliding mode crack, as shown in Fig. Shear stress attained at the interface was believed to attribute to this type of failure.
During unloading, tensile stress was built up significantly underneath the contact region and tended to lift the film from the substrate. Once the stress exceeded the interfacial bonding strength, delamination was triggered and left an opening mode crack at the interface, as shown in Fig. Abdul-Baqi A and Van der Giessen E [ 48 ] also carried out an FEM study and emphasized that tangential cracks were normally formed earlier and easier than normal cracks. In turn, if the interfacial strength was sufficiently high to prevent tensile cracks, tangential delamination would be possibly avoided.
The sample used in the study was a bilayer structure and the material properties of the film were assumed to be linear elastic. Cracking inside the layer was not taken into account. Apart from the interfacial cracks occurring, through thickness fracture or cracking inside a thin film layer is another type of failure that may occur during indenting. Such failure might be caused if a brittle thin film layer was bent into the plastically impressed substrate or the piling-up of substrate material around the indenting area [ 49 ]. In fact, for most brittle thin films under mechanical loading, through-thickness cracks are formed in conjunction with interfacial failure.
The mechanisms of the failure are very complex and have been extensively studied [ 46 , 49 , 50 , 51 ]. Li et al. In their test, a ring-shape through-thickness crack was first formed in the hard coating due to the layer toughness being insufficient to withstand the high stress formed at the contact edge.
With the progress of indentation, a second ring-like through-thickness crack was formed, in the periphery of which delamination and buckling could be found. The occurrence of delamination was coincident with the discontinuities in the P—h curves obtained from indentation. In the final stage of indentation, chipping and partial spalling occurred as a result of propagation of the secondary through-thickness crack. So far, most of the previous studies focused on bi-layer structures, which had only one layer of thin film or coating deposited on a substrate. So the structure failure was observed at the interface between film and substrate or inside a brittle thin film layer.
Liao et al. Failure in a multilayer structure is more complicated than that in a bilayer system. The FEM simulation carried out by Chen and Bull [ 45 ] shows that the stress field and distribution that determine the interfacial failure not only rely on the mechanical properties and thickness of an individual layer, but also highly depend on the penetration depth of the indenter.
As shown in Fig. The mechanisms revealed in this work applied for other similar multilayer structures [ 45 ]. In the FEM simulation of the nanoindentation on a multilayer system that consisted of a number of bulk and film layers, Zhao et al. The maximum shear stress formed during indentation was located slightly underneath the contact edge in both cases.
However, the overall stress level was reduced significantly in the layered structure. For more direct comparison purposes, the shear stress variation below the edge of contact was plotted. Given that the lateral cracking was governed by shear stresses, the study clearly demonstrated that the use of a multilayer structure could significantly increase the shear strength of a structure [ 53 , 54 ].
Fracture toughness and structural evolution in the TiAlN system upon annealing
The cross-sectional nanoindentation CSN technique was developed by Sanchez et al. CSN was designed specifically for intuitional observation of the delamination formed in a thin film multilayer structure. Prior to test, the tip was well calibrated, so one side of the triangular indentation impression was parallel to the interface, as shown in Fig. During indenting, radial cracks were generated at two corners of the Berkovich indenter if the indentation load was sufficiently high, and then propagated to the interface. When the cracks approached to the interface between the substrate and the film, they would normally change the direction and keep propagating along the interface, hence formed interfacial delamination, as shown in Fig.
In this case, chipping-off also occurred in the substrate and hence further enlarged the interfacial crack.
A jump-in observed in the load-displacement curve was associated with the interfacial cracking. Based on the linear elastic fracture mechanics and plate theory, the interfacial critical energy release rate was calculated [ 55 , 56 ]. CSN was often used for identifying the weakest interface in multilayer structures and for characterizing interfacial adhesion [ 13 , 56 , 57 , 58 ]. Compared with conventional testing methods, CSN presents some advantages. First, interfacial crack paths can be handily formed and directly examined using SEM or optical microscopy.
Second, it is applicable to variety types of multilayer structures as it can induce interfacial delamination in a controlled manner. A disadvantage of using CSN is that the method is not suitable for those substrates that do not have orientated cleavage preferential planes, as for such materials the propagation of the cracks in the substrate is hard to control. Similar to nanoindentaion, nanoscratching was valuable for investigating the failure mechanisms of thin film structures [ 67 , 68 ]. During scratching, if a ramping load is applied on the moving tip, the critical load that induces adhesion failure can be measured.
The load corresponding to complete peeling of the film is named as the higher critical load L c2 [ 50 ]. Roy et al.
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During testing, both reactive force and penetration depths were recorded. Before reaching the critical load L c1 , pile-ups were found along the edges of the scratching groove and became clearer with the increased load. The occurrence of structure failure was reflected by the discontinuities appeared on both measured scratching depth and force. Tang et al. It was interesting to note that plastic deformation was dominant over the elastic deformation prior to the failure of structure.
Recent nanoscratch studies revealed that film thickness had great effect on the critical normal load that induced interfacial failure [ 67 , 74 , 75 , 76 ]. Beake et al. The scratching results shown in Fig. The thinner the film was, the more easily through—thickness cracks occurred. This might be attributed to the higher stresses generated at the interface during scratching as thinner films provided less protection. In contrast, a thicker film would provide more support in scratching and hence more resistance to the deformation in the substrate, which in turn delayed the occurrence of interfacial delamination.
This work also suggested that tangential loading promoted the formation of lateral cracks, so structure failure in nanoscratching took place at much lower load than that in nanoindentation. Favache et al. In their study, brittle fractures on the hard film were found to be dominant during scratching, but the high elastic mismatch between two films significantly influenced the crack energy release rate and hence determined the crack density, as shown in Fig.
Nanoscratching was used to simulate an individual abrasive grit penetrating and sliding on the thin film layer surface in abrasive machining [ 24 , 66 ]. The structure of the multilayer being investigated is shown in Fig. By judiciously selecting the scratching parameters, ductile mode material removal was achieved in brittle layers, as shown in Fig. Below the critical depth, plastic deformation was dominant in the thin film layers as clear pile-ups were observed without cracks.
Evident fractures were observed when the scratching depth was above the threshold value, as shown in Fig. Multiple nanoscratches were also made on the cross-sectional surface of such a thin film multilayer to examine the effect of neighboring scratches on the surface finish [ 26 ]. The comparison of single and multi-scratched cross-sectional surfaces is shown in Fig. Under the same experimental conditions, more sever brittle fractures and interfacial delaminations were generated on the multi-scratched surfaces. To understand the interfacial failure mechanisms, the interaction of neighboring scratches and its influence on the severity of interfacial failure was systematically investigated [ 26 ].
This was done by repeating the scratching at the same location for several times. After carrying out 6 scratches, as shown in Fig. The results also demonstrated that the interaction between neighboring scratches would crush the fractured debris produced by previous scratches and hence exaggerated the interfacial failure.
FEM studies of nanoscratching on thin film structures were conducted by Li and Beres [ 77 ], aiming to explain the failure modes in scratched thin film systems. Their model used in the simulation is shown in Fig. A bilinear elastic-plastic model with isotropic strain was used to characterize the material deformation. In their simulation study, a ramping normal load was applied on the top of the indenter. Stress distribution was recorded during scratching and three stages were found at the film-indenter interface. The stages were found to be well matched with the contact modes obtained by Holmberg et al.
Tensile stress was formed behind the moving tip and was responsible for the transverse damages observed in the experiments. Compressive stress, which was the cause of film buckling, was found at the head area of the indenter. It is clear that the stress field under the indenter can be enhanced notably in the sliding process than penetration.
In the thin film multilayer structure, interfaces between dissimilar materials are susceptible to delamination or debonding when sufficient mechanically induced shear stress or tensile stress is experienced [ 79 , 80 ]. It is thus crucial to understand the stress distribution induced by scratching and how the distribution affects the interfacial failure in multilayers. Nevertheless, till now little has been reported on such understanding.
An FEM simulation was carried out to predict the stress evolution under scratching [ 26 ]. In this study, a 3D symmetrical FEM model was established to simulate the scratching process. During simulation, the input normal loads on the tip were the critical forces that induced interfacial delamination at different interfaces measured from nanoscratching tests.
Although both tensile and shear stresses formed at the interface are the potential causes for delamination, the simulation results indicated that shear stress played a more dominant role than tensile stress. The outcomes achieved offered valuable guidances for developing a high efficiency machining technology for thin film multilayer structures. Polishing and lapping were performed to study the removal characteristics of the thin film multilayer shown in Fig.
During polishing, surface damages in the glass substrate and thin films reduced with the decreased grit size. The polished surfaces shown in Fig. Crack-free surfaces indicated that ductile mode material removal was dominant during polishing. Nevertheless, the material removal rate in polishing was too low to be applied in practical application [ 81 ]. At present, there is neither a standard test procedure, nor a standalone methodology for the assessment of thin film toughness.
The determination of the toughness is still a difficult task, and very much a fully open problem. In this article, we review the hardening and toughening mechanisms of nanocomposite films, and the toughness characterization techniques. Based on these reviews, an outlook will be presented in the concluding remarks. Volume 4 , Issue 3. The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username.
Sam Zhang Corresponding Author E-mail address: msyzhang ntu.snorenpi.tk
Fracture toughness and structural evolution in the TiAlN system upon annealing
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