Micro-Impact Testing

 of Thin Films

A number of techniques are routinely used to measure the mechanical properties of wear-resistant coatings and surfaces. These include, for instance, scratch testing, indentation testing, and pin-on-disk wear testing. However, it is often difficult or impossible to correlate results obtained from such tests with actual product performance. This is particularly the case with surfaces which are subjected to erosive wear or multiple impacts, where surface toughness is of paramount importance.

A repetitive impact technique has been developed for investigating surface fracture resistance. The apparatus, which is based on the NanoTest instrument, consists of a pendulum mounted on an essentially frictionless pivot such that a fixed or variable force can be applied to a test probe and the displacement of the probe can be measured by means of a capacitive transducer.

Impact is effected by means of an oscillating piezoelectric transducer placed behind the specimen holder. This causes the probe to "bounce" on the specimen surface. The impact frequency, static load and duration of the experiment are pre-programmed. The initial and final static probe positions are determined in order to calculate the resulting depth increase, and the instantaneous probe position is monitored and plotted throughout the procedure.

In addition to the most straightforward test where the specimen is stationary, the influence of repetitive contact can be investigated as the specimen is moved underneath the impacting test probe. This allows the effects of impacts adjacent to prior impacts to be investigated to determine the influence of damage emanating from an impact site. This is compatible with many film failure situations, e.g., tool bit coatings invariably impact the workpiece at slightly different locations due to vibration. Measurements of this type are also directly related to erosive wear and permit instrumented tests of such phenomena.

Single point impact results have been obtained from a range of brittle materials which indicate an initial period of damage generation followed by catastrophic failure. Metallic specimens by contrast tend to show only indentation behaviour, as would be expected for ductile materials. In addition, in some cases where thin film delamination by scratch testing was relatively difficult, repetitive impact during specimen motion produced complete removal of the thin film from the substrate. Examples are shown below.

Abrupt impact failure of a TiN film sputter-deposited onto a Si substrate. The test probe was a 25 m m radius spherical diamond, the static load was 100 mN, and the oscillation frequency was 80 Hz. Failure was by means of a fatigue process which produced steadily enlarging circular cracks.

Abrupt failure of a TiN film sputter-deposited onto a Si substrate. In this example, the same probe, static load and frequency were employed, but the specimen was simultaneously moved beneath the probe at a speed of 100 nm/s. Scratch testing with the same probe and static load produced only slight ploughing into the TiN surface.