High Aspect Ratio Laser Machining of Composite Ceramics

R.T. Kerth IBM Storage Systems Product Division San Jose, California USA

Summary

High aspect ratio laser machining of composite ceramics has been demonstrated using a pulsed laser wavelength that is largely transparent to one of the composite constituents of the ceramic and highly absorbed in the other. Machined slots 50 microns wide and 400 microns deep are possible as the result of the deep pene-tration of the laser radiation due to the "semi-transparent" nature of the substrate. This work is presented in two parts. The first is a study of the single pulse interaction of the laser with pure samples of Alumina and separately Titanium Carbide which are the two constituents found in the machined sample. These results are compared to the single pulse interaction with the composite. Finally, results of the high aspect ratio machining on the composite are presented and a probable mechanism is proposed.

Introduction

With the development of reliable high power lasers came their application in various machining activities. In general, the machining application would match the laser wavelength to a strongly absorbing region of the material to be ma-chined (1,2). This matching for highest absorption, results in the energetics of the laser ablation process being concentrated in the top few 1000 Angstroms of the material. In these cases the machining depth does not increase linearly with the laser f1uence but saturates at some point with any increase in pulse energy merely increasing the velocity of the escaping ablated material.

If narrow (on the order of 50 microns) but deep machined grooves are desired, the near surface absorption of the material interferes with the machining opera-tion in the following way:

P. Seyfried et al. (eds.), Progress in Precision Engineering Springer-Verlag Berlin, Heidelberg 1991

203

Multiple laser pulses are overlapped with a small displacement of each succeeding pulse to form a groove. As the groove becomes deeper the ablated material does not exit from the top of the groove but deposits on the wall. The next pulse is absorbed by this debris on the side wall and is redeposited elsewhere. Of course, some of the material escapes on the subsequent pulses but the process efficiency is degraded.

In the approach presented here the laser wavelength is chosen such that the en-ergy is absorbed in the Titanium Carbide which represents 30% of the material volume. The remaining 70% of the ceramic volume is Alumina which is trans-parent to the laser wavelength. The laser pulse is absorbed by the TiC which ablates and in so doing removes the Alumina with it. In the process of making grooves as described above, each pulse penetrates several microns into the surface of this composite for efficient material removal. In addition any wall debris is now largely transparent to the next incoming pulse as it is largely Alumina and devoid of TiC.

Experimental Apparatus and Procedure

The following experiments were completed on an Electro Scientific Industries Model 44 Laser Trimming tool. This tool incorporates X-Y scanning of a pulsed YAG Laser beam. The X-Y stage has positional accuracy of 2.5 microns in both directions and can deliver a laser pulse at integral spacings of the 2.5 microns. The laser is a Q-switched Nd-Y AG at 1064 nm. with random polarization deliv-ering up to .5 mJ/pulse with a less than 35 nsec full width, half max duration.

The procedure for the initial materials investigation was to focus a 150 mJ pulse to a 35 micron spot and ablate lapped samples of Alumina, TiC, and 70%-30% "AlTiC" with single and double pulses. The reason for the double pulse was to minimize the possibility of reflection from the lapped surfaces which nominally had a one nano-meter average roughness. These pulses were then profiled, sec-tioned and photographed.

With the same laser conditions, a sample of the composite material was exposed to single train of pulses spaced 2.5 microns apart. Multiple passes over the same

204

groove were made without changing conditions. The samples were SEM'ed and sectioned.

Experimental Results

The results of the single and double pulse exposures are shown in Figures 1-3. The pulses shown are 75 microns apart and nominally 30 microns in diameter. The TiC ablation in Figure I, showed non-uniform etching with a large amount of material spatter. This may have been due to the fact that this TiC has 3 to 4% porosity which may have allowed deeper than expected penetration of the laser. Further investigation is needed.

In Figure 2, the Alumina sample tested here was hot isostatically pressed during manufacturing. This sample is very representative of the Alumina in the final composite material. This Alumina was processed under the same conditions as the TiC above. As shown in Figure 2, the Alumina did not ablate on every pulse. Those areas that did show laser ablation were shallow and very smooth compared to previous materials tested. The ablation of some spots but not others is proba-bly due to absorbing impurities near the surface of the Alumina. Generally, the fact that the Alumina did not ablate in some areas is consistent with it being transparent to this wavelength.

Finally, the ablation of the "AlTiC" exhibits uniform etching, as shown in Figure 3, with well controlled deposition of the ablated material around the top of the hole. The result is somewhat a cross between the two other materials. There is not the spattering, characteristic of the TiC but the etch pocket is not as uniform as the Alumina ablation.

From the profiled traces of each etched spot, the volume of material was calcu-lated. The result .. of these measurements are shown in Figure 4. The final column calculates the volume of TiC removed from the AlTiC by simply multiplying the total volume removed by the ratio of the TiC in the material (30%). The two numbers are within 35% of each other, which might also be accounted for by the high porosity of the TiC leading to a higher etching than lower porosity material.

205

The final experiment was to show deep narrow trenching of the composite "AITiC" material and the ability to remove debris from a very deep trench. Figure 5 shows SEM photographs of two trenches and the debris that was removed. The left trench was created using 2.5 micron spaced pulses and 25 passes were made. The right hand trench was the same condition but 5 passes were made. Figure 4 shows the sectioned grooves created in this experiment. A calculation of the area of the grooves, which is proportional to the volume removed per unit length, shows that volume of material removed per pass is reduced by only 12% for the deeper groove. This reduction may be due somewhat to side wall debris but also due to the defocusing of the beam as the ablated surface moves down into the part. In this experiment no attempt is made to re-focus the beam down into the part during the ablation.

Conclusions

Efficient machining of composite ceramics is possible by choosing a laser wave-length that is absorbed strongly by only one component of the composite. The rate of material removal of the composite is close to the rate of the pure absorbing component divided by it's volume percentage in the composite.

Acknowledgements

The author wished to extend his appreciation to M. Knight, B. Auser, Dr. W. Leung, and Dr. S. Lewis for their assistance in the preparation and analysis of the parts.

206

Figure I. Laser ablated Titanium Carbide showing spattering around a 30 micron spot that is 1.8 microns deep.

o -00

Figure 2. Laser ablation of HIP'ed Alumina showing occasional ablation due to transparent nature of the substrate.

207

Figure 3. Laser ablated composite ceramic (30% TiC, 70% Alumina) showing 2.5 microns of material removal with little spatter.

Sample Depth Width Volume Volume TiC (micron) (micron) Removed Removed

TiC 1.8 30 424 424

Alumina 1.0 32 N/A

"AI-TiC" 2.5 45 1047 314

Figure 4. Comparative ablation of three ceramic samples. Volume of ceramic ablated is calculated.

208

Figure 5. SEM photograph of high aspect ratio grooves and sectioned parts showing wall angle and depth.

References I. Chryssolouris, G; Bredt, J.; Kordas, S.; Wilson, E.; Theoretical Aspects of La-ser Machine Tool, .lour. of Engineering Industry; Feb. 88, Vol. 110.

2. ito, S; Nakamura, M; Kanematsu, W.; Machining of High Performance Ce-ramics, Bull. Japan Soc. of Prec. Eng.; Sept. 87, Vol. 21, No.3.