ABSTRACT
High power fiber-coupled diode lasers for Laser-Assisted Machining (LAM) of ceramics provides an efficient, cost effective solution for surface finishing of ceramic products. This paper presents experimental evidence of advantages of LAM over the traditional diamond wheel grinding, a standard technique currently utilized in the finishing of ceramic surfaces. LAM, utilizing fiber-coupled diode lasers, also provides advantages over other types of lasers such as CO2 and Nd:YAG lasers. The emphasis of this work is in the evaluation of LAM in the strength of finished products of two different sources of silicon nitride. An optical technique based on evanescent illumination was utilized to measure the Ra of the finished surfaces utilizing LAM, laser glazed, diamond ground, and as-received surface conditions. Four point bending test for specimens of each surface condition were utilized to measure the fracture strength. A correlation was found between the measured Ra and the predicted strengths resulting from Weibull analysis. The correlation shows a decrease of strength with the increase of Ra. The fracture surfaces were observed both optically and with a SEM, and the flaw sizes were measured. The analysis of the fractographs indicated that the flaw sizes are consistent with Fracture Mechanics predictions. Explanations of the correlation between Ra, strength, and flaw sizes require further testing.
INTRODUCTION
Over the last three decades, ceramics have moved from low strength applications to high temperature and high strength applications, based on remarkable improvements in strength, fracture toughness, and impact resistance [1, 2]. Demand for advanced ceramics is expected to increase as the infiltrate several applications cutting tools, joint implants, capacitors, military armor, aerospace, and automotive components. Advanced ceramics (silicon nitride, silicon carbide, zirconia, etc) offer higher temperature capability, lower density, higher stiffness, and better wear resistance when compared to metals [3, 4]. Traditional processing of these ceramic materials includes forming, green machining, sintering, and final machining stages. These components typically require very tight dimensional tolerances during shaping and surface machining, generally done by diamond wheel surface grinding (containing coarse, intermediate, and fine grinding stages) due to the high hardness of these ceramics. [5]. Also, conventional single point machining (turning, milling, and drilling) produces brittle failure, excessive surface damage, and excessive tool wear at acceptable machining rates [6, 7]. Therefore, the cost of machining ceramics represents 70% to 90% of the cost of finished parts [8].
Considering silicon nitride (Si3N4) as a baseline material for this study, diamond grinding still results in low material removal rates and is limited to simple contours. Prior academic research on Laser Assisted Machining (LAM) of high strength ceramics (silicon nitride, toughened zirconia, silicon carbide, etc) has included empirical studies [10-17] and numerical modeling [18-21]. This work quantifies the benefits of LAM over grinding including, rapid material removal rates and extended tool life. In LAM, the machining process is simplified and accelerated, leading to reduction in equipment cost, labor, and machining time. Applying fiber-couple diode lasers creates a more robust and industrial rugged system for LAM of ceramics.
How LAM Works
In laser assisted machining (LAM), a high energy laser beam is used to locally heat a small zone, on the workpiece, ahead of a single point tool (diamond or cubic boron nitride (CBN)) and then machined by turning or milling. By preheating a ceramic workpiece in the machining zone ahead of the tool, the local spot temperature rises to a point where quasi-ductile behavior, rather than brittle fracture occurs. Quasi-ductile deformation in the ceramic enables reduced cutting forces, high material removal rates, minimal surface damage, and increased tool life [9-11].
A schematic of the LAM process (with the ceramic rod, cutting tool, laser spot, and material removal plane) is illustrated in Figure 1. A good review of past LAM research on a variety of materials using Nd:YAG and CO2 lasers and the benefits is reported in [9].
Figure 1 Laser assisted machining model illustrating the machining removal plane.
In the case of silicon nitride, the intense heating of the surface locally raises the temperature of a glassy grain boundary phase, which is the residual of an oxide sintering aid used during liquid phase sintering at temperatures above 1300°C [4]. The composition of the sintering aid determines the temperature (~600-1200°C) at which the grain boundary phase softens. With sufficient heat, the grain boundary phase will soften and produce the desired ductile deformation. However, local overheating can introduce undesirable thermal damage such as devitrification, melting, sublimation of the grain boundary phase, or oxidation of the silicon nitride. Therefore, it is important to monitor the surface temperature profile in the cutting zone with a thermal (IR) imaging and/or 2-colour pyrometer so that material removal zone remains within the critical minimum and maximum preheat temperatures.
EXPERIMENTAL SETUP AND DISCUSSION
Material Selection
Commercial high strength silicon nitride from two independent sources was chosen as the baseline material for the LAM study and classified as silicon nitride A and silicon nitride B. Both silicon nitride sources were LAM turned in the form of 25 mm diameter rods, 150 mm long. Some tests were also completed on 13 mm by 100 mm rods of silicon nitride B. An investigation into material properties of a similar material to silicon nitride A was characterized (in a 2002 vintage) in a 2005 Army Research Laboratory study [22].
The microstructure of silicon nitride A is illustrated in Figure 2, from [22]. Note the bimodal grain size with the large acicular silicon nitride grains, the light gray grain boundary phase, and the very small white inclusions.
Figure 2 Microstructure of silicon nitride
A Laboratory & Prototype Production LAM Systems
A laboratory scale LAM system, Figure 3, was constructed at Northern Illinois University (NIU). The system setup, containing a 250W fiber-coupled diode laser, has been reported in detail in [23-25]. The system utilizes a FLIR® A325 thermal imaging camera for experimentally measuring thermal fields in the ceramic material, allowing for baseline studies of laser parameters during LAM. This academically focused bench top system is designed to assist a commercial partner in integrating LAM technology into their manufacturing operations.
Based on the laboratory LAM system, an industrial scale LAM system was designed and built at the industrial partner location using a 25 HP commercial CNC 5-axis turning center. The system utilizes a custom built multi-beam fiber-coupled diode laser system reported in [23-25]. The primary laser processing head is equipped with a built-in, coaxial, 2-colour pyrometer system (temperature range 543-1500°C) for process monitoring. Thermal imaging is also utilized for thermal measurement during part production. Dedicated hardware and software are used for data acquisition and process control.
Figure 3 NIU LAM system overview
Laser Surface Analysis
In order to validate the LAM process as a viable industrial solution, Experimental Mechanics represents a necessary tool to understanding properties and behavior of the ceramic parts after machining. Particularly, it is important to measure the surface finish produced during LAM. A recently developed optical method for characterization of the surface that was presented last year at SEM is known as Advanced Digital Moire contouring [31]. This technique utilizes evanescent illumination to interact with the surface under inspection. This is achieved through the phenomenon of light generation produced by electromagnetic resonance where the self generation of light is achieved through the use of total internal reflection (TIR). When a plane wave front impinges the surface separating two media such that the index of refraction of medium 1, glass, is higher than the index of refraction of medium 2, air (i.e. ni > n2 ), at the limit angle, total reflection takes place. Under these circumstances a very interesting phenomenon occurs at the interface (glass-air) and evanescent waves are produced. At the same time, scattered waves emanate from the medium 1 (glass). More detailed theory on this optical measurement methodology is described in [26, 30, 31].
To achieve this experimentally, a laser-microscope system, seen in Figure 4, has been constructed with a capacity to resolve vertical differences to 120 nm across a 480 x 480 micron surface area. Essentially, the surface of the ceramic is in contact with a grating that has 400 lines per mm. The surface is illuminated by a HeNe laser at an oblique angle that provides total internal reflectance. This generates a 3-D interference image captured by a CCD camera. Accuracy was calibrated against a NIST traceable surface roughness calibration block (Ra range of 3.018 – 3.079 ^m).
Figure 4 Laser Surface Analysis system setup during calibration
The interference patterns are analyzed with a fringe analysis software package, Holo Moire Strain Analyzer™ (HMSA) Version 2.0, developed by Sciammarella et. al. and supplied by General Stress Optics Inc. (Chicago, IL USA). The fringe analysis uses powerful Fast Fourier Transforms for filtering, carrier modulation, fringe extension, edge detection and masking operations, and removal of discontinuities, etc. The software produces a full statistical analysis of the interference pattern in both spread sheet and graphic form (Figure 5). Different roughness metrics like Ra, Rq, and Rz can be quickly determined and Weibull analysis used to characteristic values.
Figure 5 Laser surface analysis field of view for a) LAM turned Si3N4 sample b) As-received Si3N4 sample
Flexure Strength Testing
The 13 mm diameter rods, of silicon nitride B, were split along their axis into half rounds (5 mm thick, 12.8 mm wide, and 50 mm long), two test specimens per rod, while the 25 mm diameter, 50 mm long rods of both silicon nitride, A and B, were sliced into three arc segments (test specimens) with a larger size (5 mm thick, 19 mm wide and 50 mm long -Figure 6a). These specimens were tested in flexure with the surface condition (curved face) in tension (down) at loads less than 4000 N (880 pounds). This arc segment specimen geometry has been used for other ceramics [22] and the method (specimens, fixturing, calculations) is described in detail by Quinn [27]. The test specimens were tested in 4 point- % point bend test at room temperature in an Instron testing machine, using an articulated fixture (40 mm-20 mm spans) with tool steel roller bearings (Figure 6b). The cross head rate was 0.125 mm/min.
Figure 6 (a) Schematic of arc samples used for 4 point bend testing (b) View of experimental setup
