Cracking analysis of HVOF coatings under Vickers indentation

International Materials Reviews

Reuben

2011

This review summarises the state of knowledge on acoustic emission (AE) techniques applied to material property evaluation during indentation (e.g. hardness) testing. There are two aspects of application of AE technique to indentation which makes it unique, i.e. (1) enhancing the understanding of the evolution of material accommodation mechanisms under loading and (2) qualitative and quantitative evaluation of mechanical properties such as fracture toughness and bond strength from the AE signal. Both of these aspects have the potential to improve our understanding of the structure property relationships of current and future generation materials. In addition, the knowledge developed here can be incorporated to improve the AE based condition monitoring systems for stress critical applications. This review concentrates on the phenomena which occur during indentation and how its examination can be used to study more fundamental behaviour of materials such as deformation and fracture. The uncertainty in quantifying and measuring the total crack surface in indentation makes a simple fracture mechanics based assessment of toughness difficult. It is therefore expected that correlation between AE and fracture patterns will lead to an improved method for material's quality evaluation. The main part of this review is presented on AE of material classifications. These classifications include ceramics, glasses, composites, metals and metallic foams, thin solid films and thermally sprayed coatings. Apart from quasi-static indentation testing, attention has also been paid to studies on various AE instrumented indentation systems so that information can be derived about the progress of deformation and cracking processes. This review discusses the studies summarising those aspects that have so far been established and the areas of controversy and/or lack of knowledge. The prospect of using AE techniques to monitor indentation tests is also assessed, taking into account those few studies that have been reported so far in different groups of materials. Although with some limitations, it is concluded that AE monitored indentation testing has considerable scope to assess in much more detail the deformation and cracking properties of materials under localised stress condition. It is possible to construct empirical relationships and develop theoretical understanding linking mechanical parameters with AE signal characteristics and its derived features. However, the occurrence of multiple events at different locations superimposing the AE signal requires more advanced signal processing techniques. With the advancement of very thin films and nanomaterials, it is anticipated that AE response measured during nanoindentation will be critical for enhancing our understanding of future generation applications, as it allows individual events to be investigated without resorting to more complex signal processing techniques. In terms of material accommodation, the understanding of physical mechanisms generating AE require a multiscale approach, e...

Section: Fundamental Models Of Indentation Mechanicsmentioning

confidence: 99%

Indentation testing and its acoustic emission response: applications and emerging trends

Faisal

International Materials Reviews

Reuben

2011

“…Elastic stress fields are important when considering cracking around the indenter, and the principal normal stresses in the Boussinesq field [25] are the ones responsible for cracking. The highest compressive stress occurs in the area right beneath the indenter tip and the highest tensile stress occurs in the centre of the indentation edges and decreases along the indentation edge towards the indentation corner [26]. This suggests that edge cracks will initiate first at the surface, and will propagate along the indentation edge.…”

Section: Discussionmentioning

confidence: 99%

An improved measurement of Vickers indentation behaviour through enhanced instrumentation

Faisal¹,

Reuben²,

2010

Meas. Sci. Technol.

This work presents an enhanced instrumented Vickers indentation technique capable of recording force, displacement and acoustic emission (AE) during loading condition. Four materials were chosen for examination; copper, aluminium, steel and as-sprayed HVOF WC-12%Co coating. Results indicate that force-displacement (P-h) profiles are essentially bilinear with two characteristic slopes separated by a distinct displacement arrest for all loads above 98 N. The P-h curve indicates three distinct loading stages (I, II and III) and the stage III mechanical energy increases with loads. About 66% of the hardened steel indentations but only about 18% of the as-sprayed HVOF WC-12%Co coating indentations exhibited an AE record that could be separated into three distinct zones (A, B and C). Where zoning was possible the AE corresponding to a zone correlated well with the AE associated with a loading stage. It is concluded that plastic deformation in soft metals produced little AE, whereas brittle fracture in hardened steel and as-sprayed HVOF WC-12%Co coating produced significant AE. AE may or may not be focused onto particular stages of the indentation and a full measure of crack prevalence would require fractal dimension analysis, which is time consuming, offering a motivation for AE-based indentation testing.

“…To model more complex indent shapes, such as those produced by the Vickers indenter requires a numerical approach, such as finite element analysis [25][26].…”

Section: Introductionmentioning

confidence: 99%

“…considering elastically deformable material, ignoring surface roughness, and taking coating and substrate as solid and homogeneous materials), Baung et al [25] simulated Vickers indentation on HVOF coatings to determine the stress distributions and critical loads in the coating/substrate systems, relating their findings to the observed cracking profile. It has been indicated by Baung et al [25] that the highest compressive stress occurs in the area right beneath the indenter tip and the highest tensile stress occurs in the centre of the indentation edges and decreases along the indentation edge towards the indentation corner. This suggests that edge cracks will initiate first at the coating surface, and will propagate along the indentation edge.…”

Section: Introductionmentioning

confidence: 99%

An improved Vickers indentation fracture toughness model to assess the quality of thermally sprayed coatings

Faisal

Engineering Fracture Mechanics

Prathuru

et al. 2014

This study presents an improved approach to the quality assessment of thermally sprayed coatings. Measurements were carried out on five different coatings. Since it is the overall extent of surface cracking during Vickers indentation that is indicative of the volumetric damage, the surface crack length was measured, including the radial cracks, edge cracks, and other cracks around the indentation. It is concluded that the proposed model provides a way forward for determining the fracture toughness (K 1c ) of brittle materials where no radial cracks are developed. An elastic-plastic finite element simulation of the Vickers indentation test was conducted to locate the stress fields.