Surface Nanostructuring through a Technique Derived from Shot-­‐Peening: Recent Advances

Author:  Constance MOREL (WINOA)
Source:  The Shot Peener Magazine
Doc ID:  2014001
Year of Publication:  2014
Abstract:  
Over the past decades, ultrafine-grained materials have attracted Considerable scientific interests, especially nanocrystalline materials, whose grain size inferior to 100 nm conveys superior mechanical, physical, and chemical properties compared to conventional coarse-grained materials. It is well known that most of failures of engineering materials (such as fatigue fracture, fretting fatigue, wear, corrosion, etc) are very sensitive to the structure and properties of material surface, and in most cases material failures occur on the surface. As a consequence, a material exhibiting a nanostructured surface is expected to be much less likely to undergo such damages without changing the chemical compositions. Many techniques have been developed to achieve surface nanocrystallization. Referred to as Severe Plastic Deformation (SPD) techniques, they all rely on the plasticity of metals and lead to a mechanically induced nanostructuration of the surface. Some of them can be referred to as “bulk treatments” as they aim at transforming the whole volume of a part They include ECAP (Equal Channel Angular Pressing), HPT (High Pressure Torsion) or drilling. On the other hand, some techniques focus on the surface: SMAT (Surface Mechanical Attrition Treatment) or USSP (Ultrasonic Shot Peenng), ball milling, sliding wear, While those processes were developed at laboratory scale and are hardly compatible with up-scaling for application to mass-production, Severe Plastic Deformation and the resulting surface nanostructuring can also be achieved through a technique derived from Shot-Peening called NanoPeening®. In most cases, Shot-Peening carried out longer or stronger than usually leads to “overpeening”, which is detrimental to the material, inducing cracks and surface degradation without any change in the microstructure. In the last few years it was discovered that it is possible, under specific conditions, to pass over these effects and reach a nanocrystallised state of the surface. NanoPeening®-type treatments thus allow for the fast generation of a thick layer (several tens of micrometers) characterized by a gradient in grain size from a nanocrystalline microstructure at the surface to the conventional core structure. Like all SPD techniques, the nanostructured layer is produced “in-situ”, i.e. without any external addition like a coating, but this new treatment differs from the processes mentioned earlier in that it offers a real potential in industrial applications with high productivity, reliability and reproducibility. It is also very flexible, as a large range of shapes, sizes and steel grades can be treated. This article aims at giving a description of NanoPeening® process, including the modeling works carried out with Pr. Guagliano’s team from Politecnico di Milano: using a Finite Elements method, they developed a program that allows for an estimation of the nanostructured layer created by the treatment, for a given set of process parameters.