Micro-impact Testing Research Articles

Nano- and Micro-Scale Impact Testing of Hard Coatings: A Review

In this review, the operating principles of the nano-impact test technique are described, compared and contrasted to micro- and macro-scale impact tests. Impact fatigue mechanisms are discussed, and the impact behaviour of three different industrially relevant coating systems has been investigated in detail. The coating systems are (i) ultra-thin hard carbon films on silicon, (ii) DLC on hardened tool steel and (iii) nitrides on WC-Co. The influence of the mechanical properties of the substrate and the load-carrying capacity (H3/E2) of the coating, the use of the test to simulate erosion, studies modelling the nano- and micro-impact test and performing nano- and micro-impact tests at elevated temperature are also discussed.

Micro-scale impact resistance of coatings on hardened tool steel and cemented carbide

Micro-impact, a novel accelerated test method for assessing coating durability under repetitive contact, has been developed to concentrate impact-induced stresses close to the interfaces in coating systems. Test results are described for carbon coatings on hardened tool steel and nitride-based coatings on cemented carbide. At higher load it was possible to show the increasing contribution of the substrate properties (load support and ductility) to the coating system response whilst retaining high sensitivity to the coating properties. Hard and elastic carbon coatings on hardened tool steel displayed very low impact resistance under these conditions. Relatively soft carbon-based coatings with more metallic character and high plasticity (low H/E) deposited on hard but tough tool steel were resistant to radial cracking and lateral fracture at high load. Lateral fracture at high load and extensive substrate cracking was observed at higher load for hard nitrides on cemented carbide. The micro-impact test has the potential to significantly speed up the pace of coating system selection for durability under highly loaded repetitive contacts, as occur in coatings applications in engine components and in discontinuous cutting operations.

Enhancing fatigue wear resistance of a bulk metallic glass via introducing phase separation: A micro-impact test analysis

Bulk metallic glasses (BMGs) are potential candidate materials for numerous structural applications. However, this class of materials has a major limitation for engineering applications due to their inferior resistance to fatigue wear. In present work, a novel micro-scale repetitive impact test was utilized for fatigue testing of ZrCu-based BMGs (Zr63.6-xCu18Ni10.4Al8Fex, x = 0, 3, 5 at.%), and specific attention was paid to the micro-alloying effect on wear resistance. Transmission electron microscopy of as-cast Zr58.6Cu18Ni10.4Al8Fe5 revealed amorphous structure with the formation of a second glassy phase of few nanometers in diameter. Although these structural changes have minor effects on hardness and modulus, they have a substantial influence on fatigue wear resistance. Phase separation was found to promote numerous shear bands, which induced strain hardening, and improved the crack resistance. Consequently, all these features are considered to contribute the improved wear resistance. On the contrary, a monolithic glass without Fe addition lacks any microstructural features that can provide means for the local arrest of shear bands and incipient fatigue cracks. These findings improve the understanding of the impact fatigue mechanisms and provide suggestions for the future design of BMGs with excellent wear performance.

Elevated temperature micro-impact testing of TiAlSiN coatings produced by physical vapour deposition

A high temperature micro-impact test has been developed to assess the fracture resistance of hard coatings under repetitive dynamic high strain rate loading at elevated temperatures. The test was used to study the temperature dependence of the resistance to micro-scale impact fatigue of TiAlSiN coatings on cemented carbide at 25–600 °C. Nanoindentation and micro-scratch tests were also performed over the same temperature range. The results of the micro-impact tests were dependent on the impact load, coating microstructure, coating and substrate mechanical properties, and their temperature dependence. At higher temperatures there was a change in failure mechanism from fracture-dominated to plasticity-dominated behaviour under the cyclic loading conditions. This was attributed to coating and substrate softening.

Micro-impact testing of AlTiN and TiAlCrN coatings

A novel micro-scale repetitive impact test has been developed to assess the fracture resistance of hard coatings under dynamic high strain rate loading. It is capable of significantly higher impact energies than in the nano-impact test. It retains the intrinsic depth-sensing capability of the nano-impact test enabling the progression of the damage process to be monitored throughout the test, combined with the opportunity to use indenters of less sharp geometry and still cause rapid coating failure. The micro-impact test has been used to study the resistance to impact fatigue of Al-rich PVD nitride coatings on cemented carbide. The impact fatigue mechanism has been investigated in nano- and micro-scale impact tests. Coating response was highly load-dependent. A Ti0.25Al0.65Cr0.1N coating with high H3/E2 performed best in the nano- and micro- impact tests although it was not the hardest coating studied. The role of mechanical properties, microstructure and thickness on impact behaviour and performance in cutting tests is discussed.

Identifying the stress–strain curve of materials by microimpact testing. Application on pure copper, pure iron, and aluminum alloy 6061-T651

Abstract

Development of an improved method for identifying material stress–strain curve using repeated micro-impact testing

Impact-based Mechanical Surface Treatments such as shot peening are widely used in aerospace, nuclear and other industries to improve the mechanical resistance of components. Measuring the stress–strain curve of materials under high-strain rate using repeated impacts is a key issue to improve such processes. This study presents an extension of a method developed by Kermouche (2013) for identifying the material stress–strain curve. It combines numerical and experimental approach using micro-impact testing. The main originality of the present work is the use of the impact load values instead of the depth of the residual imprint as an input parameter of the inverse identification. The reliability of the proposed method is then checked from a set of numerical blind tests. A direct method derived from Tabor’s pioneering work (Tabor, 2000) is also proposed to convert the impact measurements into an approximate stress–strain curve. These two methods have been applied on a commercially pure copper and show very good agreement. The main advantage of this analysis is to determine the mechanical behaviour of metallic surface at high strain rate using limited numbers of samples and tests.

Local identification of the stress–strain curves of metals at a high strain rate using repeated micro-impact testing

Understanding the effect of various mechanical processes such as shot peening requires the knowledge of metal behavior at a high strain rate. The identification of this behavior is often performed using Hopkinson's bar devices. However, the resulting stress–strain curves correspond to a bulk behavior of the material and thus do not take into account the modification induced by surface preparations or surface treatments. In this paper, we have proposed a method based on local micro-impact testing to identify the stress–strain curves of metals near their surface. More precisely, this method is based on the determination of the best stress–strain curve which allows to reproduce the growth of the residual imprint at each impact for a given impact energy. The advantage of the method developed in this paper lies in its simplicity and low cost. In the first part, the repeated impact set-up is presented. Then the FEM strategy is detailed and the identification method developed. In the last part of this paper, an application to AISI1045 and AISI316L steels highlights the great interest of this method to obtain better stress–strain curves than those classically used. It points out the need to identify appropriate surface stress–strain curves when the surface behavior is concerned. As a matter of fact it should also concern other manufacturing processes such as machining or finishing.

A HIGH-SPEED MICRO-IMPACT TESTING SYSTEM FOR EVALUATING THE PROPERTIES OF LEAD-FREE SOLDER JOINTS

This study presents a novel high-speed micro-impact testing system for evaluating the shear strength and energy absorbance properties of lead-free solder ball joints under impact velocities of 0.3–1.0 m/s. The test method can examine the failure mode transition from ductile failure to brittle failure based on the failure mode distribution percentage. The load—displacement curves can distinguish between different failure modes. Notably, the values of peak load and the energy-to-peak load at various impact speeds can be used as indices for determining solder ball under impact loading conditions. Thus, the proposed method is a suitable technique for evaluating the mechanical response of solder ball joints used in typical semiconductor packages and, therefore, represents a simple and effective technique for optimizing the chip-level reliability of portable consumer electronic products.

Short cycle hardening and tempering behaviour of low alloy tool steel 60WCrV7

AbstractThe effect of short heat treatment cycles, typical of induction or laser hardening, on the low alloy steel 60WCrV7, was simulated using a quench dilatometer. The heat treated material was examined using optical and scanning electron microscopy, microhardness testing, X-ray diffraction and microimpact testing. The effects of variation in austenitising time and temperature as well as tempering parameters on the hardness and toughness obtained are reported, and the metallurgical reasons for this behaviour are discussed. The results provide conclusions for optimum rapid cycle heat treatments and reveal significant potential for strongly reduced heat treatment cycles, which may have applications in areas besides surface hardening.

Investigating the fracture resistance and adhesion of DLC films with micro-impact testing

Micro-impact testing, a new repetitive contact testing technique has been developed to simulate the fatigue conditions which coatings experience in service more closely than existing testing methods. Diamond-like carbon (DLC) films deposited on silicon under different RF powers have been investigated with the new technique. The technique provides a new approach to the investigation of the fracture properties of thin films. Single-point impact data for brittle materials, such as DLC films, show an initial period of damage generation followed by catastrophic failure. The time-to-failure and the overall change in probe depth during a test are measures of the resistance of the coating to fracture. Differences in impact resistance between the DLC films are explained in terms of variations in film toughness and adhesion strength, which, in turn, are a function of the DLC deposition conditions.

Micro-impact testing: a new technique for investigating fracture toughness

A novel micro-impact technique has been developed to simulate the repetitive contact conditions which many thin films and coatings experience in service. In this paper we describe how the technique has been used to investigate the impact resistance of ceramic coatings of almost identical microhardness. There were very clear differences in impact behavior between the coatings, which correlated with measurements of their fracture toughness determined from crack inspection. Coatings with low fracture toughness values exhibited high wear rates and a marked susceptibility to brittle fracture. The high surface roughness of some of the ceramic samples precluded the determination of critical loads by scratch testing. In contrast, the micro-impact technique is less affected by sample roughness than scratch testing, and therefore, is able to provide useful data on a wider range of materials. The micro-impact technique offers an important new method for the determination of the fracture toughness of thin films where methods involving the direct visualization of cracks are impossible.

Micro-Impact Testing: A New Technique for Investigating Thin Film Toughness, Adhesion, Erosive Wear Resistance, and Dynamic Hardness

A small scale probe testing method has been developed for investigating the behaviour of thin films under dynamic loading conditions. The primary objective of the development was to produce quantifiable techniques that closely simulate the conditions that thin films experience in service. Variations of the technique allow measurements related to: impact wear and adhesion failure, erosive wear resistance, fracture toughness, work hardening, and dynamic hardness. The common element in each variant is the acceleration of a test probe (usually diamond) towards the specimen surface and the monitoring its instantaneous position before and after collision. The impact energy can be controlled and either single impacts or multiple impacts can be produced. For single impacts, the energy delivered to the contact point can be quantified, allowing calculation of a dynamic hardness number.

Micro-impact technique and its applications

A micro-impact tester has been designed and built by using a piezoelectric impact hammer as an impactor driver. During the course of an impact process, force interactions between the impactor and target surfaces were monitored continuously by a miniaturized piezoelectric loadcell embedded in the flying head assembly. A 3-sided pyramidal diamond indenter (Berkovich indenter) was used as an impactor. After having fully characterized such an impact system, the trajectory of the impactor can be calculated by using the corresponding prerecorded impact force interaction in a simulation program. The contact and returning velocities, kinetic energy loss of the impactor, and the impact penetration curve are the key pieces of information obtained from the simulation. Furthermore, the impact morphology can reveal failure mechanisms of materials by providing details such as indent shapes, coating fragment, chipping, crack type and size, and other information which are useful in assessing the fracture toughness of testing materials. The micro-impact testing was carried out in the contact velocity ranging from 0.3 to 2.0 m/s. Three types of materials such as metal, glass, and amorphous carbon were used in studying their distinct mechanical behavior under high rate indentations. The correlations among the impact conditions, energy losses, impact morphologies, and material responses are illustrated and discussed.

Micro-Impact Technique and its Applications

ABSTRACTA micro-impact tester has been designed and built by using a piezoelectric impact hammer as an impactor driver. During the course of an impact process, force interactions between the impactor and target surfaces were monitored continuously by a miniaturized piezoelectric loadcell embedded in the flying head assembly. After having fully characterized an impact system, the trajectory of the impactor can be calculated by using the corresponding pre-recorded impact force interaction in the simulation program. The contact and returning velocities, kinetic energy loss of the impactor and the impact penetration curve are the key information obtained from the simulation. Furthermore, the impact morphology can reveal failure mechanisms of materials by providing details such as indent shapes, coating fragments, chipping, crack type and size, and other such information which are useful in assessing the fracture toughness of testing materials. The micro-impact testing was carried out in the contact velocity ranging from 0.3 to 2.0 m/sec. Three types of materials such as metal, glass and amorphous carbon were used in studying their distinct mechanical behavior under high rate indentations. The correlations between the impact conditions, energy losses, impact morphologies and material responses are illustrated and discussed.