Micromachining of AI 2 O 3T 1 C ceramics by excimer láser

Micromachining of AI2O3-TÍC ceramic using a KrF excimer láser was studied in the fluence range 2 to 8 J/cm . The ablation rate decreases and the roughness increases with the first pulses but after about 200 pulses the process reaches a stationary stage where both roughness and ablation rate become constant. Observation of the processed áreas by scanning electrón microscopy showed that a globular topography is formed during the first stage and that the surface topography remains unchanged with further pulses. This globular topography is responsible for the variation of roughness and ablation rate observed during the first stage. EDS analysis showed that the globular features present an external región with higher titanium contení and a core formed of unaffected material.


INTRODUCTION
Láser micromachining presents characteristics that are particularly well adapted to the production of miniaturised components of complex shape, such as magnetic head sliders.In the present paper we report results of a study on láser micromachining of Al 2 0 3 -TiC using a KrF excimer láser.
ning 66 mass % A1 2 0 3 and 34 mass % TiC.The radiation source was a KrF Lambda Physik LPX 305 láser, operating at 248 nm, with pulse length of approximately 20 ns.Details of the experimental procedures were given in previous publications (1 and 2).After irradiation, the topography of the láser processed áreas was investigated by optical profilometry and scanning electrón microscopy (SEM).High resolution SEM and chemical analysis were performed using a field emission gun SEM with energy dispersive spectrometry (EDS) attachment.

RESULTS
Figure 1 shows the variation of the removal depth per pulse with the number of pulses for láser fluences of 2 and 8 J/cm 2 .As can be seen, there is a 206 significant fall in the removal rate with increasing number of pulses, until it reaches a constant valué after 200 pulses.This valué increases with higher láser fluence.
The roughness of the machined surfaces first increases with increasing number of pulses and then reaches a constant valué after approximately 200 pulses (Fig. 2).The roughness also increases with increasing láser fluence.
The SEM micrograph in figure 3 shows the surface morphology of áreas irradiated with 50 and 200 pulses at 6 J/cm 2 .After processing with 50 láser pulses, globular features appear at the processed surface.After 200 láser pulses the surface is completely covered by these globules and the mean globule size increases.A further increase in the number of pulses up to 1.000 does not change the surface morphology significantly.The growth of this láser modified layer is certainly responsible for the variation observed in both removal rate and roughness.The microstructure of the surface layer, revealed in fractured globules (Figs.4-8), consists of an unaltered internal (core) región (Fig. 5), and a featureless titanium rich outer layer (Fig. 7).

DISCUSSION
It was shown in a previous paper (3) by AES and EPMA analysis, that the globules which formed after láser irradiation are rich in titanium, carbón and oxygen, and that the región between globules is rich in aluminium.It was suggested that a differential ablation meehanism was responsible for the formation of the globules.In the present paper it is shown that each globule presents two different regions: an external layer rich in titanium, in agreement with the results obtain by EPMA and AES, and an internal región or core consisting of two different phases, one rich in titanium and another rich in aluminium, similar to the material in pristine condition.
These new results reinforce the conclusión that a differential ablation meehanism is responsible for the observed topography.A higher ablation rate for alumina, possibly because of an electronic ablation meehanism for excimer láser ablation of alumina, as suggested by other authors (4-6), would leave a TiC rich layer behind, and a lower ablation rate of this phase would act as a shield for the underlying material.A differential meehanism of this kind would accordingly explain the external layer rich in titanium, and the unaltered core.

CONCLUSIONS
1.The roughness and the removal rate of A1 2 0 3 -TiC ceramic samples irradiated in air with 2 to 8 J/cm 2 KrF excimer láser light depend strongly on the number of láser pulses up to 200 pulses, then become constant.The ablation rate in stationary regime is 0.015 and 0.032 ¡Ltm/pulse at 2 and 8 J/cm 2 respectively.
2. Removal rate and roughness increase with láser fluence.The limiting roughness increases in the range 0.12 to 0.65 mm when the fluence increases from 2 to 8 J/cm2.3. The variation of roughness and removal rate is due to the formation of a globular surface layer.Each globule of this layer presents an external layer rich in titanium and an internal región presenting two different phases, one rich in titanium and another rich in aluminium.