Scratch testing under progressive loading (PLST) is a wide-spread engineering method for mechanical strength asessment (adhesive and cohesive strength!) of coated materials. So far the almost exclusive quality criteria retained are critical loads for cohesive and adhesive coating-failure (LCl, LC2) as directly detected off-line by scratch track inspection under a microscope and frequently on-line, indirectly, by means of abrupt variations in acoustic emission (AE) and/or friction force (FF).
For practical friction and wear applications under cyclic (essentially elastic) loading, a major detrimental hard coating failure mechanism is brittleness, with mode I (through thickness) cracks, once generated, likely to propagate in mode II along the substrate/coating interface. For medium to high strength coatings (LCl, LC2 > 40N) quality ranking, based exclusively on critical loads of spalling, is often too severe and sometimes even misleading with respect to tribological "field applications" The unambiguous assessment of first cracking failure is thus a key to scientifically backed up laboratory quality ranking with a good predictive potential as to surface mechanical field applications, and thus a prerequisite basis for efficient written standards.
For both AE and FF there are sofar no instruments available allowing for online detection of minor cracking events The present work is meant to overcome this shortcoming by introducing high speed AE detection and video monitoring of the ontact zone in the stress applied state (because of closing up after unloading, small cracks will also pass undetected under the microscope). Thus the capabilities of the well known engineering scratch test can be considerably extended and upgraded. The objective of this project is to pave the way for the establishment of really meaningful critical first cracking failure loads, finally likely to be clearly related to failure mechanisms under elastic loading in friction fatigue of wear protective hard coatings. The introduction of multimode operation will also allow to unambiguously establish this cross link on the same instrument.
The work of the present programme will be carried out in ten distinct tasks:
Task 1 : Specimen procuremcnt,
Task 2 : Multimode operation software development,
- High load indentation (lNcL<200N),
- Single pass conventional PLST (lN - Single pass operation at constant loads,
- Multipass operation at constant sub-critical loads,
Task 3 : Acoustic emission upgrade for single crack event detection, Task 4 : Crack identification by means of acoustic microscopy and standard metallography,
Task 5 : On-line optical sensor development for continuous damage monitoring in the stress applied state,
Task 6 : High speed and high resolution video development for continuous on-line storage of optical frames (triggered by validating AE pulses) and analog signals (pertinent system parameters) via a buffer to a mas storage system,
Task 7 : Video an analog data exploitation,
Task 8 : Assembling of the integrated system (prototypes)
Task 9 : Inter comparison,
Task 10: Establishment of instructional leaflets and video as well as guideline for standardisation
The present programme is meant to lead to a major break-through in scratch test instrumentation which has been practically stagnant over at least ten years. With very sensitive on-line detection, validation and monitoring of brittle cracking coating failure, in addition to availability of dynamic operation modes, there is a high probability that this up-graded technique will finally lead to the quality criteria relevant for practical applications involving friction and wear.
Funding SchemeCSC - Cost-sharing contracts
DY10 4JB Kidderminster,hartlebury
5595 AN Leende