An analysis of the quality of experimental design and reliability of results in tribology research

In recent years several high profile projects have questioned the repeatability and validity of scientific research in the fields of psychology and medicine. In general, these studies have shown or estimated that less than 50% of published research findings are true or replicable even when no breaches of ethics are made. This high percentage stems from widespread poor study design; either through the use of underpowered studies or designs that allow the introduction of bias into the results. In this work, we have aimed to assess, for the first time, the prevalence of good study design in the field of tribology. A set of simple criteria for factors such as randomisation, blinding, use of control and repeated tests has been made. These criteria have been used in a mass review of the output of five highly regarded tribology journals for the year 2017. In total 379 papers were reviewed by 26 reviewers, 28% of the total output of the journals selected for 2017. Our results show that the prevalence of these simple aspects of study design is poor. Out of 290 experimental studies, 2.2% used any form of blinding, 3.2% used randomisation of either the tests or the test samples, while none randomised both. 30% repeated experiments 3 or more times and 86% of those who repeated tests used single batches of test materials. 4.4% completed statistical tests on their data. Due to the low prevalence of repeated tests and statistical analysis it is impossible to give a realistic indication of the percentage of the published works that are likely to be false positives, however these results compare poorly to other more well studied fields. Finally, recommendations for improved study design for researchers and group design for research group leaders are given

Defining the role of 'zero wear volume' in percussive impact

This work defines the previously undetermined contribution of the ‘zero wear’ volume, that is geometry change due to material compression that occurs before other mechanisms that cause change through actual material loss are initiated during to repetitive impact. Five metal alloys widely used in engineering applications, each with a different bulk hardness, were the subjects of the experiments. Using a reciprocating hammer type impact wear test apparatus, flat coupon type specimens were subjected to repetitive impact from a chrome steel ball acting normal to the surface. 36,000 impacts were applied at a nominal rate of 10 impacts per second, each with an impact energy of 0.23J and an impact force of 3.5 kN. The impact wear crater on selected worn specimens was examined using a 3D non-contact profilometer. Scanning electron microscopy techniques were used to further examine the damage on the specimens. The main damage mechanism was plastic deformation and surface fatigue due to spalling. Microcracks and adhered wear debris were noted on the specimens, but with no evidence of delamination, while subsurface examination showed no possible microcracks under the impacted surface and only surface pitting could be observed from subsurface examination. Analysis of the wear scars suggests that zero wear volume is the main contributor to the total volume ‘loss’ for all materials, and, for specific materials, plastic flow volume and bulk hardness could be a significant parameter in characterising zero-wear volume and crater depth

Analysis of reciprocating hammer type impact wear apparatus

Repetitive impact wear of metals is caused by primary wear mechanisms removing material from the surfaces and is responsible either solely, or in part, for the failure of engineering components (e.g. automotive valvetrains, wheel-rail contacts, mining equipment). Fundamental work exists, but overwhelmingly considers the impact wear of a particular material or surface treatment. The mechanisms involved are variously oxidative, adhesion, abrasion, surface fatigue and plastic deformation. The dominance is controlled by stress, sliding conditions, impact energy, and the difference in material properties, particularly those of plastic deformation, between the two surfaces. There is a paucity of published work that solely investigates and characterises the fundamentals of impact wear. Impact wear test apparatus falls into two groups; projectiles propelled into a stationary target/specimen, or a target/specimen being repeatedly struck by a hammer. The former type is similar to that commonly used for erosive wear, so the reciprocating hammer/striker type design, was chosen for analysis here. Analysis was performed on specimens with impact wear scars that feature both material displacement and material loss, enabling comparison between different measurement techniques to be made. This provides a protocol for easier comparison of data, promoting improved models, and furthers the understanding of the apparatus’ performance