Correlation between Mechanical Strength and Surface Conditions of Laser Assisted Machined Silicon Nitride Part 2

EXPERIMENTAL RESULTS AND DISCUSSION

Design of Experiments

The room temperature flexural strength of LAM machined Si3N4 specimens were investigated, in both silicon nitrides, against surface conditions as listed in Table 1. All specimens were prepared on the prototype production LAM system. While, process parameters remain proprietary, please note that baseline multi-beam LAM parameters were used, and are not optimized for material removal rates, tool wear, or surface conditions. Ongoing and future work aims at process optimization.

A two-parameter Weibull analysis was used to determine variability of strength and surface roughness for each sample. In ceramics, the Weibull distribution is used to characterize strength behavior on the basis that the weakest link in the body will control the strength, as described by Quinn [28].

Table 1 Specimens considered for flexure testing and surface characterization

Silicon Nitride Material A		Silicon Nitride Material B
25 mm Dia., 50 mm Long Arc Segments		25 mm Dia., 50 mm Long Arc Segments
Surface Condition	# of Samples Tested	Surface Condition	# of Samples Tested
As-Received	9	As-Received	9
Diamond Ground (100 Grit)	9	Diamond Ground (800 Grit)	9
LAM Turned	9	LAM Turned	9
		Laser Glazed	9
		13 mm Dia., 50 mm Long Half Rounds
		As-Received	6
		Diamond Ground (800 Grit)	6

Statistical Static Properties

Weibull analysis of flexure strength and surface roughness, relating to the 27 tested specimens in silicon nitride A, was previously reported in [25]. Results indicated that LAM did show an increase in Weibull strength and lower variance over the as-received condition, with the exception of one LAM test rod (three arc segment tests). Optical and SEM fractography was done on the high and low strength rods from Source A and flaw sizes measured. Further investigation into the effect of laser surface heating, laser glazing, is highlighted in Silicon nitride B testing and reported in [32]. Results lead to further confirm an increase in Weibull strength, with lower variance, during LAM, as well as laser glazed conditions over as-received conditions.

Surface Roughness

Surface measurements, Ra, of the 36 silicon nitride B (from twelve 25 mm dia. rods) and 27 silicon nitride A test specimens consisted of taking three different area measurements per specimen, 27 areas per rod, for a total of 81 area measurements per surface condition. From the Weibull distribution, the characteristic roughness value, Ra0 with 95% confidence, along with the upper and lower bound (UB and LB) values of Ra0 for the two silicon nitrides is found in Table 2.

Table 2 Weibull analysis of surface roughness measurements, Ra0

	Silicon Nitride B				Silicon Nitride A
	As-Received	Diamond Ground (800 Grit)	LAM Turned	Laser Glazed	As-Received	Diamond Ground (100 Grit)	LAM Turned
Ra0 (^m)	1.2378	0.9339	0.887	0.9485	1.260	1.336	0.956
UB of Ra0 (^m)	1.4509	1.034	1.0407	1.0052	1.619	1.544	1.067
LB of Ra0 (^m)	1.0561	0.8434	0.7588	0.8949	0.981	1.156	0.856

Weibull analysis of LAM surface roughness, for both silicon nitrides, showed improvement over the corresponding as-received conditions, while the coarser 100 grit diamond ground produced the highest roughness measurements of all samples considered. LAM and laser glazed conditions are found to have comparable surface finishes to that of the 800 grit diamond ground.

Flexure Strength

The silicon nitride A, 27 wide (19 mm), specimens fractured across a range of applied loads – 1905 to 3950 N (427 to 885 lbs). While silicon nitride B, 36 wide (19 mm), specimens fractured across a range of applied loads -2433 to 3960 N (545 to 887 lbs) and the 12 narrow (12 mm) specimens fractured across a range of applied loads -1679 to 3504 N (376 to 785 lbs). All of the specimens fractured within the inner span and took generally about 100140 seconds. Tables 3 & 4 shows the characteristic strength, c0, along with its UB and LB and mean flexure strength along with the coefficient of variation (CoV) and extreme values (high and low) for the four surface conditions and both silicon nitrides.

Table 3 Comparison of Weibull strength & statistical strength values in Silicon Nitride B testing

25 mm Dia. Rods					13 mm Dia. Rods
	As-Received	Diamond Ground (800 Grit)	LAM Turned	Laser Glazed	As-Received	Diamond Ground (800 Grit)
c0 (MPa)	452.1	526.5	582.0	547.9	N/A	N/A
UB, LB of c0 (MPa)	476, 429	575, 483	611, 554	575, 522	N/A	N/A
Mean Strength (MPa)	436.3	497.9	560.5	528.2	318.1	516.4
CoV – Mean Strength	8.10%	13.20%	9.00%	8.50%	5.00%	18.10%
High, Low Strength (MPa)	499, 388	624, 430	623, 469	588, 458	338, 296	620, 388
Number of Samples Tested	9	9	9	9	6	6

Table 4 Comparison of Weibull strength & statistical strength values in Silicon Nitride A testing

25 mm Dia. Rods
	As-Received	Diamond	LAM	LAM
		Ground	Turned	Turned
		(100 Grit)	Rods #5&6	Rod #4
o0 (MPa)	549.0	488.0	609.8	416.35
UB, LB of G0 (MPa)	584, 517	514, 463	623, 597	481, 361
Mean – Strength (MPa)	524.0	469.0	599.2	390
CoV – Strength	11.60%	9.30%	4.9%	16.5%
High, Low Strength (MPa)	604, 415	531, 408	623, 541	447, 300
Number of Samples Tested	9	9	6	3

Further analysis of the flexure strength data has shown, experimentally, a correlation with an increase in surface roughness, a decrease in flexure strength occurs in both sources of Si3N4 material, as seen in Figure 7. Along with the observed trend, the laser glazed condition also demonstrates a decrease in surface roughness and an increase in flexure strength over the as-received condition. It would appear, as with the LAM turned specimens, that the laser heating may have a healing effect on surface flaws. More investigation into these trends is required and further discussed in the following sections.

Figure 7 Weibull strength, og, vs. Weibull surface roughness, Rag, for all surface conditions tested Fracture Mechanics

NIST recommended practice guide by Quinn [28] for the fractography of ceramics is followed. Images of the fracture surfaces were captured both optically and by SEM (gold coated) for sample specimens selected from silicon nitride A testing. Selected samples highlight the low strength LAM (three test specimens), high strength LAM (three test specimens), and as-received conditions. Selected samples for silicon nitride B testing, highlight the various surface conditions and all values reported are averaged per condition, at least three test specimens. Fractographs are also post processed using the HMSA software package, taking advantage of light contrast over the fractured surface, to aid in visualizing the fracture mirror, Figure 7.

Fracture Toughness Estimation

Approximations of fracture toughness (KIc), given in Tables 5 & 6, are estimated based on a technique and (1) described by Quinn [28]. The flaw size (a and c are shown in Figure 8) and shape are estimated by inspection of optical and SEM fractographs. Additionally, the maximum stress intensity factor, Y, is found as the Newman-Raju Y factor at the surface and deepest part of the crack [28]. The Newman-Raju Y factors are also included in ASTM standard C 1421 for fracture toughness of ceramics. The Y factor at the surface is considered for discussed during comparisons. Values for strength, cf, were measured experimentally during silicon nitride A & B testing. These calculations and interpretations are not conclusive as further investigation of these findings, in more detail, is underway.

Table 5 Estimation of KIc for highlighted surface conditions of Si3N4 A by descending strength values

Surface Condition SiN A	Strength, MPa	Crack Depth, ^m	Ydepth	Kicd Mpa*m12	Ysurface	Klics Mpa*m12
LAM (Low Strength)	390	140.3	1.62	7.08	1.25	5.51
As- Received (#7B)	604	63.0	1.55	7.43	1.24	5.92
LAM (High Strength)	619	183.6	1.53	12.82	1.21	10.10

Table 6 Estimation of K_Ic for Highlighted surface conditions of Si₃N₄ by descending strength values

Surface Condition SisN4 B	Strength, MPa	Crack Depth, ^m	Ydepth	KIcd Mpa*m12	Ysurface	KIcs Mpa*m12
As-Received (Ave.)	431	245.7	1.55	10.11	1.17	7.73
800 Grit DG (Ave.)	464	181.6	1.58	9.87	1.17	7.33
Laser Glazed (Ave.)	538	148.7	1.61	10.40	1.16	7.59
LAM Turned (Ave.)	595	213.5	1.59	13.80	1.16	10.01

The manufacturer specified fracture toughness for silicon nitride A and B is reported as 6.3 MPa*m12 and 6.1 MPa*m12, respectively, which is close to that estimated in the as-received conditions. While low values of KIc are estimated for one LAM case in silicon nitride A, Figure 8; inherent material flaws, machining damage, mishandling, or specimen preparation (cutting of arc segments) may have resulted in undesired strengths. On the other hand, laser effected samples of both materials (higher estimated KIc in LAM) appears to further support the surface healing effect of laser heating. Laser glazing of alumina has been reported to similarly improve Weibull strength characteristics over as-received conditions [29]. These results indicate that fully understanding the mechanism warrants additional in-depth study.

Fracture Origin

Figure 8 Fracture Origin in low strength LAM #4A shown during post processing, highlighting the fracture mirror.

Figure 9 Fracture origin in low strength LAM #4A a) optical photo of the fracture mirror shows crack of about 136 jxm deep, and combined with a 447.2 MPa fracture strength and a surface Y factor of ~1.27 gives KIc ~ 6.65 MPa*m1/2. b) SEM image offracture mirror showing LAM turned surface.

CONCLUSIONS

Various test specimens prepared by laser-assisted machining, laser glazing, conventional diamond grinding, and as-received surface conditions were evaluated on silicon nitride rods from two different sources. During the LAM process it was found that LAM turned silicon nitride rods had a measured 25%-30% increase in flexure strength, when compared to their as-received counterparts. For test specimens prepared by laser glazing an increase in strength and decrease in surface roughness, over as-received conditions, was also observed. Experimental results from both material sources have demonstrated an apparent trend correlating the surface roughness and flexure strength for all surface conditions tested.

Initial fractography of the laser heated specimens along with the as-received condition was completed. In one case, LAM indicates reduced strength and fracture toughness compared to as-received conditions. While this result was uncharacteristic of all other LAM samples tested, care must be taken while machining and handling the ceramic so as to avoid degrading the strength aside from surface induced flaws. Results also indicate laser heating of the ceramic surface may in fact increase the strength of the ceramic material . While further investigation is required to confirm this estimation, the increase in strength may be a result of a modification of the flaw sizes and flaw populations found in as-received counterparts, acting in a beneficial way. LAM provides a very promising, cost effective, improvement over conventional diamond grinding of advanced ceramics.