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Characterization and machining of alumina ceramic reinforced with lanthanum phosphate
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
journal homepage: www.elsevier.com/locate/jmatprotec
Characterization and machining of alumina ceramic reinforced with lanthanum phosphate M. Abdul Majeed a,∗ , L. Vijayaraghavan a , S.K. Malhotra b , R. Krishnamoorthy a a b
Department of Mechanical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India Composite Technology Centre, Indian Institute of Technology, Madras, Chennai 600036, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
Increasing demand for materials for severe working environment necessitates the use of
Received 5 May 2007
ceramics. Fiber/whisker reinforcement in the structure can lead to certain defects such
Received in revised form
as debonding/delamination. Hence one can resort to particulate reinforcements. Of late
25 May 2008
attempts have been made to introduce dispersion strengthened/particulate reinforced
Accepted 31 May 2008
ceramic composites. Addition of lanthanum phosphate (LaPO4 ) and cerium phosphate (CePO4 ) to alumina (Al2 O3 ) matrix has been attempted. In this study Al2 O3 /LaPO4 composites containing different LaPO4 content have been assessed for the significance of LaPO4 content
Keywords:
on structure-property and consequent machinability. Ultrasonic drilling trials have been
Characterization
carried out. The response of the material to the machining environment has been assessed
Ultrasonic
by monitoring the acoustic emission (AE) from the composites and defects induced during
Solid tool
machining. © 2008 Elsevier B.V. All rights reserved.
Hollow tool Composites Ultrascan
1.
Introduction
Ceramics with their higher hardness, stability and lower density find increasingly high engineering applications; their applicability is constrained by their lower order fracture toughness and thermal shock resistance which pose serious machining problems. The allowable normal load in machining ceramics is related to structural properties by n
Fall ˛ (k1c ) 1 (E/H) an3
n2
where, Fall is the allowable normal load; k1c is fracture toughness; E is Young’s modulus; H is hardness; a is depth of penetration; n1 , n2 , n3 are the constants. The relatively lower value of k1c and E/H calls for restricted depth of penetration. This is seen in diamond turning of ceramic material, involving ductile regime machining of brittle
∗
Corresponding author. Tel.: +91 44 22574687; fax: +91 44 22575705. E-mail address: [email protected] (M.A. Majeed). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.05.049
material. Researches concerning enhancement of machinability of ceramic materials have been attempted. Apart from the concept of ductile regime machining, attempts for limiting the normal force during cutting is mostly concerned with structural modification. Addition of certain inclusion favoring dispersion strengthening of ceramic materials can facilitate easier machining by way of introduction of weaker interfaces, crack arrest/crack bridging mechanisms. Researches on relatively harder ceramic composites containing rare-earth materials such as LaPO4 and CePO4 have been attempted. Davis et al. (1998) reported that the ease of machining increased with increasing volume fraction of the rare-earth phosphate component. Wang et al. (2002, 2003a,b) have reported that the sinterability and hardness of Al2 O3 /LaPO4 composites are deeply dependant on the LaPO4 content. Luo et al. (2004) have reported that Al2 O3 reinforced with 30% Ti3 SiC2
2500
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
could be machined using conventional Fe–Mo–W drills. Ultrasonic machining is preferable for ceramic materials. In the case of ultrasonic machining, since the total cutting force is small, the process does not induce mechanical stresses sufficient to cause warping or other residual deformation. Also the heating is negligible if the process is operated properly. Hence the structure is not affected (Rozenberg et al., 1964). The present work focuses on the effect of LaPO4 content on the Al2 O3 /LaPO4 composites while undergoing ultrasonic drilling. The performance was evaluated in terms of material removal rate and other material response related indicators.
Table 1 – Specifications of AE sensor Model Make Peak sensitivity, dB ref. 1 Vm/s (1 V/mbar) Operating frequency range (kHz) Resonant frequency (kHz) Directionality (dB)
RSWD Physical Acoustical Corporation 50 [−74] 200–1000 250 [525] ±1.5
the machining environment was evaluated through the power spectrum of the AE signal monitored and corresponding rms value.
2.
Experimental procedure
2.3.
2.1.
Specimen preparation
Hardness of the work materials was measured by using Microhardness Tester using different loads varying from 200 to 2000 g (dwell time = 10 s). Eight readings were taken for each specimen for each load. The microstructure of the work materials was studied using scanning electron micrographs (SEM). Ultrascan (C-scan) of drilled specimens was carried out by Pulse-Echo-immersion method at a frequency of 25 MHz to obtain a clear picture of the internal defects. X-ray diffraction was done on XD-D1-X-ray diffractometer by using Cu K␣ radiation and the intensity profiles were an analyzed for Al2 O3 /LaPO4 composites of varying compositions.
Al2 O3 /LaPO4 composite powder of different LaPO4 content (pure alumina, 30 wt.% LaPO4 , 50 wt.% La PO4 and pure LaPO4 ) were ball-milled for 24 h, dried using rotary vaporizer and cold pressed at 150 MPa and then sintered at 1600 ◦ C for 2 h and cooled down slowly to room temperature (size of alumina particles was 0.3–0.6 m and LaPO4 was in the form of nano-clay with diamond structure of 100 nm × 1 nm or 2 nm).
2.2.
Ultrasonic machining
Drilling of holes was done on LE HELDT-DIATRON IC, Ultrasonic drilling machine with a frequency range of 18–22 KHz and a power output of 0.9 KW. The abrasive used was Boron Carbide (Grit size-280) mixed with water in the ratio 1:3 and with a static load of 500 g. Tools used were 3 mm diameter solid and hollow tools with central hole of 1 mm diameter (low carbon steel tools). Machining time was noted in each case. Acoustic emission (AE) from the work piece was monitored by using a broad band sensor. The piezoelectric sensor was suitably positioned behind the work holder to pick-up the signals, amplified using a 160B-type pre-amplifier and further transferred to the AET5500 signal conditioning system, connected to the computer. The specification of the sensor is given in Table 1. The response of the work material to
Characterization
3.
Results and discussion
3.1.
Hardness
Typical monitored variation of microhardness of Al2 O3 /LaPO4 composites against percentage of LaPO4 is shown in Fig. 1(a). It is seen that with increased LaPO4 content a gradual reduction in hardness can be seen up to 30% beyond which hardness appreciably drops down. The observed reduction in hardness with LaPO4 is reflected in the increasing order of material removal rate (MRR). However it is to be noted that the MRR drops down beyond 50% despite the observed appreciable reduction in hardness. Wang et al. (2003a) have reported that the addition of LaPO4 forms a fine stable LaPO4 phase which
Fig. 1 – Variation of hardness.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
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Fig. 2 – Scanning electron micrographs. (a) Alumina, (b) LaPO4 , (c) Al2 O3 /LaPO4 (70:30) composite and (d) Al2 O3 /LaPO4 (50:50) composite.
refines the grain size of Al2 O3 and results in segregating Al2 O3 grain boundary. The presence of weaker interface and grain boundary segregation could have resulted in crack deflection inducing enhanced machinability of the ceramic composite, however the refinement of Al2 O3 with higher order LaPO4 content could have resulted in the observed reduction in MRR. Fig. 1(b) illustrates the typical observed variation of Vicker’s hardness with indenter load. It is seen that Al2 O3 and Al2 O3 /LaPO4 (70:30) composite exhibit a progressive increase in hardness with indenter load while only a marginal variation in hardness with indenter load can be seen in LaPO4 and ceramic composites with higher order LaPO4 content. Wang et al. (2003b) have reported that the sinterability of composites reduced with LaPO4 tending to a decline in the mechanical/physical properties of composites.
3.2.
grains. The compact presents a close-packed denser structure. The compact also exhibits localized smeared/glazed texture due to sintering. Micrograph of sintered Al2 O3 /LaPO4 (70:30) ceramic composite is illustrated in Fig. 2(c). Unlike the case of Al2 O3 , alumina ceramic composite containing 30% of LaPO4 shows well marked grain network, attributable to refinement of alumina grains due to the addition of LaPO4 . Micrograph of sintered Al2 O3 /LaPO4 ceramic composite containing higher LaPO4 content (50% by weight) is shown in Fig. 2(d). Unlike the case of 70:30 ceramic composites, 50:50 Al2 O3 /LaPO4 sintered composite presents a structure with discrete smeared/glazed zones, indicating reduced order of structural compatibility. i.e., excess LaPO4 settles down discretely, resulting in reduced order of sinterability.
Microstructure 3.3.
Typical micrograph of sintered Al2 O3 is illustrated in Fig. 2(a). It is seen that the grains are of acicular morphology, which is characteristic of brittle materials. The illustration presents a good grain boundary sintered structure with few isolated porous sites. Micrograph of sintered LaPO4 is illustrated in Fig. 2(b). It is seen that the grains are elongated, exhibiting layered sub-
Material removal rate
Machinability of ceramic composites can be assessed in terms of MRR, tool wear and also other relative indirect indicators. Typical observed MRR as influenced by LaPO4 content is illustrated in Fig. 3. It is seen that up to certain percentage content of LaPO4 , a higher order MRR occurs, beyond which a reduction sets in. It is seen that the trend of variation in MRR with
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
Fig. 3 – Variation of MRR percentage of LaPO4 .
LaPO4 content, tends to change over with 30%, marginally, and appreciably with 50%. Up to 30% LaPO4 content, there is a progressive rise in MRR, followed by a slower increase in MRR with further increase in LaPO4 content. The observed variation in microhardness with indenter load is also supporting the observed slower increase in MRR with higher order LaPO4 content.
Ultrasonic drilling is associated with impingement of harder abrasives on the work piece at higher frequency (=20 KHz). Consequently the material is removed by matrix erosion due to momentum transfer and occasionally due to cavitation induced in the tool–work gap (due to high frequency oscillations of the tool). Hence hardness of the work material will be the prime factor in deciding the machinability. Normally higher the work piece hardness, better will be the machining, in terms of the geometrical accuracy of the hole drilled. However with increased fracture toughness (k1c ), ceramic composite can experience better machining by way of crack-free machining. This is illustrated in Fig. 3. Up to 30% LaPO4 , a progressive rise in machining rate can be seen, attributable to the critical value of LaPO4 content on structural compatibility. This is supplemented through the scanned images of drilled holes. It is to be noted that, despite the reduction in hardness with LaPO4 content, only a marginal variation in MRR is seen above 50% LaPO4 addition. This supports the concept of hardness required for ultrasonic drilling. With hollow tool, appreciable enhancement in MRR can be seen. The continuous erosion facilitated by the central suction of the annular tool has resulted in improved abrasion action and thus the improved MRR. During ultrasonic drilling, an abrasive slurry is introduced between the work piece and an oscillating tool. The tool material is usually a softer steel. The abrasive slurry falls on the oscillating tool with the consequent hammering of the abrasive on the work piece. The
Fig. 4 – C-scan. (a) LaPO4 , (b) Al2 O3 /LaPO4 , (c) Al2 O3 /LaPO4 (70:30) and (d) alumina.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
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Fig. 5 – X-ray diffraction patterns. (a) Alumina specimen, (b) alumina (machined powder), (c) Al2 O3 /LaPO4 (70:30) specimen, (d) Al2 O3 /LaPO4 (70:30) machined powder, (e) Al2 O3 /LaPO4 (50:50) specimen, (f) Al2 O3 /LaPO4 (50:50) machined powder, (g) LaPO4 specimen and (h) LaPO4 machined powder.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
tool can be solid or annular section. With the solid tool, as the tool penetrates into the work piece, the effective end erosion drops down. With hollow tool, the abrasive flow is maintained well (associated with central suction) enhancing the material removal rate appreciably as seen in the illustration.
3.4.
Ultrascan (C-scan)
Typical observed scanned images of holes drilled in Al2 O3 and Al2 O3 /LaPO4 composites are illustrated in Fig. 4(a–d). In the C-scans, the side bar shows the amplitude of ultrasound emissions and the X- and Y-axes show the linear scale in ‘mm’. It is seen that in pure Al2 O3 , severe damage occurs around the hole periphery resulting in reduced machining. With increased LaPO4 content, due to grain refinement and also
possible crack deflection, machining rate improves as seen by relatively clear scan image in the case of 70:30 composite. With higher order LaPO4 content (50%), rounding of the hole boundary can be seen in the scan images. In plain LaPO4 , fluffing of the material around the hole boundary can be seen due to entry shock (lack of fracture toughness). Referring to the illustrations in Fig. 4, it is seen that, better machining has been obtained with 70:30 composite, compared to other proportions, by way of reduced defects induced during machining.
3.5. Phase analysis—X-ray diffraction (XRD) intensity profile observation The test material was evaluated through XRD analysis for the phase content. Typical XRD intensity profile for sintered alu-
Fig. 6 – AE power spectra for solid tool. (a) Alumina, (b) LaPO4 , (c) Al2 O3 /LaPO4 (70:30) and (d) Al2 O3 /LaPO4 (50:50).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
2505
Fig. 6 – (Continued ).
mina is illustrated in Fig. 5(a). It is seen that the alumina contains dominant peaks on Bragg’s angle of 25.7◦ , 35.3◦ , 43.5◦ and 57.6◦ . During machining, the machined chips (powder) were collected and corresponding X-ray intensity profile was obtained. Fig. 5(b) illustrates typical XRD intensity pattern of machined alumina powder. The strain-induced state of the powder can be seen in the peak broadening (reduced intensity) and also shift in the peaks, indicating mixed mode of stress induction; some are tensile stresses while others are in compressive mode. This can be attributed to differential stress induction during sintering of the alumina ceramic.
Typical observed XRD profile for Al2 O3 (70:30) composite in the as-sintered condition is illustrated in Fig. 5(c). Compared to the XRD profile of sintered alumina, the alumina peaks in the case of Al2 O3 /LaPO4 sintered composite has shifted (increased) with increasing Bragg’s angle indicating compressive residual stresses due to sintering of composite. This has resulted in the observed progressive increase in hardness with indenter load up to 1000 g with Al2 O3 /LaPO4 composite, unlike Al2 O3 , which exhibits a critical load of 500 g. The induction of compressive residual stresses due to LaPO4 has resulted in enhanced critical load and MRR. The observed XRD profile for machined 70:30 ceramic composite powder is illustrated in Fig. 5(d). The relaxation of the
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
induced compressive stresses in the as-sintered Al2 O3 /LaPO4 (70:30) has resulted in the relatively sharper peaks in XRD profile. The enhanced MRR observed is reflected in the observed relaxation of sintering stresses. Fig. 5(e) illustrates the typical observed XRD profile for 50:50 Al2 O3 /LaPO4 ceramic composite. It is seen that Al2 O3 peaks undergo a peak shift and broadening indicating lattice/phase straining possibly due to refinement of the alumina at higher order LaPO4 content. Typical XRD profile of machined powder Al2 O3 /LaPO4 (50:50) ceramic composite is shown in Fig. 5(f). It can be seen that machined powder of 50:50 composite is richer in Al2 O3 content compared to 70:30 machined powder. This can be attributed to possible reduced order sinterability with higher order LaPO4 content. The relatively softer LaPO4 could have fallen off. Referring to the XRD intensity profile of the machined powder, it is seen that peaks pertaining to 70:30 composites exhibits more relaxation compared to plain alumina or 50:50 proportion. This attributes to the better sintering status with 70:30 proportion. Typical XRD intensity profile of sintered LaPO4 specimen and machined LaPO4 powder chips are illustrated in Fig. 5(g) and (h). As in the earlier cases, XRD profile of machined powder chips indicate induced material stressing with mixed mode, attributable to the condition of the sintered LaPO4 .
3.6.
Acoustic emission analysis
Response of the ceramic composites to ultrasonic drilling has been assessed by monitoring the AE signal emitted from the work piece. AE is relatively a weak elastic stress wave emitted by a material under stress and thus it is a reliable indicator of the status of the material. The signal usually of higher frequency and smaller amplitude is picked up by a broad band piezoelectric sensor and amplified further for signal analysis. Typical power spectra of the signals monitored are illustrated in this section. Power spectrum of AE signal monitored during the entry of the tool during ultrasonic drilling of alumina is illustrated in Fig. 6(a). The signal is of mixed mode with dominant peaks over 50–100 KHz and 250–300 KHz frequency range. During tool entry, the work material experiences fluffing associated with continuous erosion and cracking of material. The observed higher frequency peaks can be attributed to cracking of the surface material during entry. With depth of penetration, the erosion will be steady as reflected in relatively reduced amplitude of peaks over 50–100 KHz. With progressive depth of penetration a mild increase in the power of AE signal monitored can be seen. As depth of penetration increases, the effective thickness of the work piece to be drilled, reduces resulting in the observed rise in the AE signal power. The signal monitored at the exit exhibits higher power owing to exit fluffing. In drilling of LaPO4 , owing to its lower hardness, the power of AE signal monitored drops down, as indicated by relatively lower power emitted (Fig. 6(b)). With depth of penetration there is a steady drop in the power as indicated by reduction in power of the signal monitored. At the exit, a
Fig. 7 – Variation of AE rms for solid tool.
marginal rise in power can be seen in the signal containing low frequency peaks. Drilling of Al2 O3 /LaPO4 (70:30) ceramic composite exhibits relatively enhanced performance as indicated by the absence of any burst mode (high frequency peaks). The reinforcement of Al2 O3 and LaPO4 could have arrested the cracking tendency facilitating better machining. With increase in depth of penetration, the machining rate is more or less maintained as indicated by a marginal reduction in peak amplitude (Fig. 6(c)). At exit, the signal monitored does not contain any high frequency peaks indicating absence of any cracking/fluffing tendency. The nature of AE signal monitored in the case of 70:30 ceramic composite indicates improved and sustained machining. In the case of Al2 O3 /LaPO4 composite of 50:50 composition, relatively low power AE signal (Fig. 6(d)) was monitored, attributable to the reduced hardness of the composite and reduced order of sinterability with higher LaPO4 content. Typical observed variation of rms values of the acoustic emission signal monitored during ultrasonic drilling of Al2 O3 /LaPO4 composite is illustrated in Fig. 7. It is seen that alumina releases high power AE signal at entry and exit as indicated by the higher magnitude of rms values. Drilling of LaPO4 results in more or less steady emission as illustrated by only a marginal variation in AE rms value. This is supported by the observed marginal variation in hardness with the indentation load. 70:30 Al2 O3 /LaPO4 ceramic composite exhibits a relatively reduced power of emission at the tool entry, followed by a marginal rise in power of emission with depth of penetration of the tool. This is attributed to the better quality of sintering and dispersion strengthening associated with 70:30 composition. With 50:50 composition, a wide variation of AE rms values can be seen, attributable to poor sintering quality of the composite.
4.
Conclusions
From the observation on ultrasonic drilling of Al2 O3 /LaPO4 ceramic composites, the following major conclusions are drawn.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
• The microhardness decreased gradually with increase in LaPO4 content up to 30% and beyond this, hardness dropped appreciably indicating the limiting value of the addition for structural compatibility. • The hardness of the Al2 O3 /LaPO4 composites showed a progressive increase with indenter load when LaPO4 content was low, but with higher LaPO4 content, only a marginal variation in hardness was observed, indicating poor sinterability. • As the LaPO4 content increased, machinability also increases. However with higher order LaPO4 content a reduction in MRR sets in, indicating the reduced applicability of higher proportion to ultrasonic drilling. • Monitoring of acoustic emission indicates the response of the composite material to ultrasonic drilling in terms of power spectrum and rms value. • X-ray diffraction indicated the presence of compressive residual stresses due to sintering of composites and relaxation of the stresses induced in the machined composites. • In all cases, with hollow tool, appreciable enhancement in MRR is observed. The study indicates that the addition of LaPO4 up to 30% can be attempted for enhanced sinterability and machinability of the composites.
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Acknowledgement The authors would like to thank the Ceramics Division, Regional Research Laboratory, Thiruvananthapuram, India, for providing the specimens for this study.
references
Davis, J.B., Marshall, D.B., Housley, R.M., Morgan, P.E.D., 1998. Machinable ceramics containing rare-earth phosphates. Journal of American Ceramic Society 81 (8), 2169–2175. Luo, Y., Li, S., Pan, W., Chen, J., Wang, R., 2004. Machinable and mechanical properties of sintered Al2 O3 –Ti3 SiC2 composites. Journal of Materials Science 39, 3137–3140. Rozenberg, L.D., Kazantsev, V.F., Makarov, L.O., Yakhimovich, D.F., 1964. Ultrasonic Cutting. Consultants Bureau, New York. Wang, R., Pan, W., Chen, J., Fang, M., Meng, J., 2002. Effect of LaPO4 content on the microstructure and machinability of Al2 O3 /LaPO4 composites. Materials Letters 57, 822–827. Wang, R., Pan, W., Chen, J., Jiang, M., Luo, Y., Fang, M., 2003a. Properties and microstructure of machinable Al2 O3 /LaPO4 ceramic composites. Ceramics International 29, 19–25. Wang, R., Pan, W., Chen, J., Fang, M., Jiang, M., Cao, Z., 2003b. Microstructure and mechanical properties of machinable Al2 O3 /LaPO4 composites by hot pressing. Ceramic International 29, 83–89.
journal homepage: www.elsevier.com/locate/jmatprotec
Characterization and machining of alumina ceramic reinforced with lanthanum phosphate M. Abdul Majeed a,∗ , L. Vijayaraghavan a , S.K. Malhotra b , R. Krishnamoorthy a a b
Department of Mechanical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India Composite Technology Centre, Indian Institute of Technology, Madras, Chennai 600036, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
Increasing demand for materials for severe working environment necessitates the use of
Received 5 May 2007
ceramics. Fiber/whisker reinforcement in the structure can lead to certain defects such
Received in revised form
as debonding/delamination. Hence one can resort to particulate reinforcements. Of late
25 May 2008
attempts have been made to introduce dispersion strengthened/particulate reinforced
Accepted 31 May 2008
ceramic composites. Addition of lanthanum phosphate (LaPO4 ) and cerium phosphate (CePO4 ) to alumina (Al2 O3 ) matrix has been attempted. In this study Al2 O3 /LaPO4 composites containing different LaPO4 content have been assessed for the significance of LaPO4 content
Keywords:
on structure-property and consequent machinability. Ultrasonic drilling trials have been
Characterization
carried out. The response of the material to the machining environment has been assessed
Ultrasonic
by monitoring the acoustic emission (AE) from the composites and defects induced during
Solid tool
machining. © 2008 Elsevier B.V. All rights reserved.
Hollow tool Composites Ultrascan
1.
Introduction
Ceramics with their higher hardness, stability and lower density find increasingly high engineering applications; their applicability is constrained by their lower order fracture toughness and thermal shock resistance which pose serious machining problems. The allowable normal load in machining ceramics is related to structural properties by n
Fall ˛ (k1c ) 1 (E/H) an3
n2
where, Fall is the allowable normal load; k1c is fracture toughness; E is Young’s modulus; H is hardness; a is depth of penetration; n1 , n2 , n3 are the constants. The relatively lower value of k1c and E/H calls for restricted depth of penetration. This is seen in diamond turning of ceramic material, involving ductile regime machining of brittle
∗
Corresponding author. Tel.: +91 44 22574687; fax: +91 44 22575705. E-mail address: [email protected] (M.A. Majeed). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.05.049
material. Researches concerning enhancement of machinability of ceramic materials have been attempted. Apart from the concept of ductile regime machining, attempts for limiting the normal force during cutting is mostly concerned with structural modification. Addition of certain inclusion favoring dispersion strengthening of ceramic materials can facilitate easier machining by way of introduction of weaker interfaces, crack arrest/crack bridging mechanisms. Researches on relatively harder ceramic composites containing rare-earth materials such as LaPO4 and CePO4 have been attempted. Davis et al. (1998) reported that the ease of machining increased with increasing volume fraction of the rare-earth phosphate component. Wang et al. (2002, 2003a,b) have reported that the sinterability and hardness of Al2 O3 /LaPO4 composites are deeply dependant on the LaPO4 content. Luo et al. (2004) have reported that Al2 O3 reinforced with 30% Ti3 SiC2
2500
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
could be machined using conventional Fe–Mo–W drills. Ultrasonic machining is preferable for ceramic materials. In the case of ultrasonic machining, since the total cutting force is small, the process does not induce mechanical stresses sufficient to cause warping or other residual deformation. Also the heating is negligible if the process is operated properly. Hence the structure is not affected (Rozenberg et al., 1964). The present work focuses on the effect of LaPO4 content on the Al2 O3 /LaPO4 composites while undergoing ultrasonic drilling. The performance was evaluated in terms of material removal rate and other material response related indicators.
Table 1 – Specifications of AE sensor Model Make Peak sensitivity, dB ref. 1 Vm/s (1 V/mbar) Operating frequency range (kHz) Resonant frequency (kHz) Directionality (dB)
RSWD Physical Acoustical Corporation 50 [−74] 200–1000 250 [525] ±1.5
the machining environment was evaluated through the power spectrum of the AE signal monitored and corresponding rms value.
2.
Experimental procedure
2.3.
2.1.
Specimen preparation
Hardness of the work materials was measured by using Microhardness Tester using different loads varying from 200 to 2000 g (dwell time = 10 s). Eight readings were taken for each specimen for each load. The microstructure of the work materials was studied using scanning electron micrographs (SEM). Ultrascan (C-scan) of drilled specimens was carried out by Pulse-Echo-immersion method at a frequency of 25 MHz to obtain a clear picture of the internal defects. X-ray diffraction was done on XD-D1-X-ray diffractometer by using Cu K␣ radiation and the intensity profiles were an analyzed for Al2 O3 /LaPO4 composites of varying compositions.
Al2 O3 /LaPO4 composite powder of different LaPO4 content (pure alumina, 30 wt.% LaPO4 , 50 wt.% La PO4 and pure LaPO4 ) were ball-milled for 24 h, dried using rotary vaporizer and cold pressed at 150 MPa and then sintered at 1600 ◦ C for 2 h and cooled down slowly to room temperature (size of alumina particles was 0.3–0.6 m and LaPO4 was in the form of nano-clay with diamond structure of 100 nm × 1 nm or 2 nm).
2.2.
Ultrasonic machining
Drilling of holes was done on LE HELDT-DIATRON IC, Ultrasonic drilling machine with a frequency range of 18–22 KHz and a power output of 0.9 KW. The abrasive used was Boron Carbide (Grit size-280) mixed with water in the ratio 1:3 and with a static load of 500 g. Tools used were 3 mm diameter solid and hollow tools with central hole of 1 mm diameter (low carbon steel tools). Machining time was noted in each case. Acoustic emission (AE) from the work piece was monitored by using a broad band sensor. The piezoelectric sensor was suitably positioned behind the work holder to pick-up the signals, amplified using a 160B-type pre-amplifier and further transferred to the AET5500 signal conditioning system, connected to the computer. The specification of the sensor is given in Table 1. The response of the work material to
Characterization
3.
Results and discussion
3.1.
Hardness
Typical monitored variation of microhardness of Al2 O3 /LaPO4 composites against percentage of LaPO4 is shown in Fig. 1(a). It is seen that with increased LaPO4 content a gradual reduction in hardness can be seen up to 30% beyond which hardness appreciably drops down. The observed reduction in hardness with LaPO4 is reflected in the increasing order of material removal rate (MRR). However it is to be noted that the MRR drops down beyond 50% despite the observed appreciable reduction in hardness. Wang et al. (2003a) have reported that the addition of LaPO4 forms a fine stable LaPO4 phase which
Fig. 1 – Variation of hardness.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2499–2507
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Fig. 2 – Scanning electron micrographs. (a) Alumina, (b) LaPO4 , (c) Al2 O3 /LaPO4 (70:30) composite and (d) Al2 O3 /LaPO4 (50:50) composite.
refines the grain size of Al2 O3 and results in segregating Al2 O3 grain boundary. The presence of weaker interface and grain boundary segregation could have resulted in crack deflection inducing enhanced machinability of the ceramic composite, however the refinement of Al2 O3 with higher order LaPO4 content could have resulted in the observed reduction in MRR. Fig. 1(b) illustrates the typical observed variation of Vicker’s hardness with indenter load. It is seen that Al2 O3 and Al2 O3 /LaPO4 (70:30) composite exhibit a progressive increase in hardness with indenter load while only a marginal variation in hardness with indenter load can be seen in LaPO4 and ceramic composites with higher order LaPO4 content. Wang et al. (2003b) have reported that the sinterability of composites reduced with LaPO4 tending to a decline in the mechanical/physical properties of composites.
3.2.
grains. The compact presents a close-packed denser structure. The compact also exhibits localized smeared/glazed texture due to sintering. Micrograph of sintered Al2 O3 /LaPO4 (70:30) ceramic composite is illustrated in Fig. 2(c). Unlike the case of Al2 O3 , alumina ceramic composite containing 30% of LaPO4 shows well marked grain network, attributable to refinement of alumina grains due to the addition of LaPO4 . Micrograph of sintered Al2 O3 /LaPO4 ceramic composite containing higher LaPO4 content (50% by weight) is shown in Fig. 2(d). Unlike the case of 70:30 ceramic composites, 50:50 Al2 O3 /LaPO4 sintered composite presents a structure with discrete smeared/glazed zones, indicating reduced order of structural compatibility. i.e., excess LaPO4 settles down discretely, resulting in reduced order of sinterability.
Microstructure 3.3.
Typical micrograph of sintered Al2 O3 is illustrated in Fig. 2(a). It is seen that the grains are of acicular morphology, which is characteristic of brittle materials. The illustration presents a good grain boundary sintered structure with few isolated porous sites. Micrograph of sintered LaPO4 is illustrated in Fig. 2(b). It is seen that the grains are elongated, exhibiting layered sub-
Material removal rate
Machinability of ceramic composites can be assessed in terms of MRR, tool wear and also other relative indirect indicators. Typical observed MRR as influenced by LaPO4 content is illustrated in Fig. 3. It is seen that up to certain percentage content of LaPO4 , a higher order MRR occurs, beyond which a reduction sets in. It is seen that the trend of variation in MRR with
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Fig. 3 – Variation of MRR percentage of LaPO4 .
LaPO4 content, tends to change over with 30%, marginally, and appreciably with 50%. Up to 30% LaPO4 content, there is a progressive rise in MRR, followed by a slower increase in MRR with further increase in LaPO4 content. The observed variation in microhardness with indenter load is also supporting the observed slower increase in MRR with higher order LaPO4 content.
Ultrasonic drilling is associated with impingement of harder abrasives on the work piece at higher frequency (=20 KHz). Consequently the material is removed by matrix erosion due to momentum transfer and occasionally due to cavitation induced in the tool–work gap (due to high frequency oscillations of the tool). Hence hardness of the work material will be the prime factor in deciding the machinability. Normally higher the work piece hardness, better will be the machining, in terms of the geometrical accuracy of the hole drilled. However with increased fracture toughness (k1c ), ceramic composite can experience better machining by way of crack-free machining. This is illustrated in Fig. 3. Up to 30% LaPO4 , a progressive rise in machining rate can be seen, attributable to the critical value of LaPO4 content on structural compatibility. This is supplemented through the scanned images of drilled holes. It is to be noted that, despite the reduction in hardness with LaPO4 content, only a marginal variation in MRR is seen above 50% LaPO4 addition. This supports the concept of hardness required for ultrasonic drilling. With hollow tool, appreciable enhancement in MRR can be seen. The continuous erosion facilitated by the central suction of the annular tool has resulted in improved abrasion action and thus the improved MRR. During ultrasonic drilling, an abrasive slurry is introduced between the work piece and an oscillating tool. The tool material is usually a softer steel. The abrasive slurry falls on the oscillating tool with the consequent hammering of the abrasive on the work piece. The
Fig. 4 – C-scan. (a) LaPO4 , (b) Al2 O3 /LaPO4 , (c) Al2 O3 /LaPO4 (70:30) and (d) alumina.
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Fig. 5 – X-ray diffraction patterns. (a) Alumina specimen, (b) alumina (machined powder), (c) Al2 O3 /LaPO4 (70:30) specimen, (d) Al2 O3 /LaPO4 (70:30) machined powder, (e) Al2 O3 /LaPO4 (50:50) specimen, (f) Al2 O3 /LaPO4 (50:50) machined powder, (g) LaPO4 specimen and (h) LaPO4 machined powder.
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tool can be solid or annular section. With the solid tool, as the tool penetrates into the work piece, the effective end erosion drops down. With hollow tool, the abrasive flow is maintained well (associated with central suction) enhancing the material removal rate appreciably as seen in the illustration.
3.4.
Ultrascan (C-scan)
Typical observed scanned images of holes drilled in Al2 O3 and Al2 O3 /LaPO4 composites are illustrated in Fig. 4(a–d). In the C-scans, the side bar shows the amplitude of ultrasound emissions and the X- and Y-axes show the linear scale in ‘mm’. It is seen that in pure Al2 O3 , severe damage occurs around the hole periphery resulting in reduced machining. With increased LaPO4 content, due to grain refinement and also
possible crack deflection, machining rate improves as seen by relatively clear scan image in the case of 70:30 composite. With higher order LaPO4 content (50%), rounding of the hole boundary can be seen in the scan images. In plain LaPO4 , fluffing of the material around the hole boundary can be seen due to entry shock (lack of fracture toughness). Referring to the illustrations in Fig. 4, it is seen that, better machining has been obtained with 70:30 composite, compared to other proportions, by way of reduced defects induced during machining.
3.5. Phase analysis—X-ray diffraction (XRD) intensity profile observation The test material was evaluated through XRD analysis for the phase content. Typical XRD intensity profile for sintered alu-
Fig. 6 – AE power spectra for solid tool. (a) Alumina, (b) LaPO4 , (c) Al2 O3 /LaPO4 (70:30) and (d) Al2 O3 /LaPO4 (50:50).
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Fig. 6 – (Continued ).
mina is illustrated in Fig. 5(a). It is seen that the alumina contains dominant peaks on Bragg’s angle of 25.7◦ , 35.3◦ , 43.5◦ and 57.6◦ . During machining, the machined chips (powder) were collected and corresponding X-ray intensity profile was obtained. Fig. 5(b) illustrates typical XRD intensity pattern of machined alumina powder. The strain-induced state of the powder can be seen in the peak broadening (reduced intensity) and also shift in the peaks, indicating mixed mode of stress induction; some are tensile stresses while others are in compressive mode. This can be attributed to differential stress induction during sintering of the alumina ceramic.
Typical observed XRD profile for Al2 O3 (70:30) composite in the as-sintered condition is illustrated in Fig. 5(c). Compared to the XRD profile of sintered alumina, the alumina peaks in the case of Al2 O3 /LaPO4 sintered composite has shifted (increased) with increasing Bragg’s angle indicating compressive residual stresses due to sintering of composite. This has resulted in the observed progressive increase in hardness with indenter load up to 1000 g with Al2 O3 /LaPO4 composite, unlike Al2 O3 , which exhibits a critical load of 500 g. The induction of compressive residual stresses due to LaPO4 has resulted in enhanced critical load and MRR. The observed XRD profile for machined 70:30 ceramic composite powder is illustrated in Fig. 5(d). The relaxation of the
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induced compressive stresses in the as-sintered Al2 O3 /LaPO4 (70:30) has resulted in the relatively sharper peaks in XRD profile. The enhanced MRR observed is reflected in the observed relaxation of sintering stresses. Fig. 5(e) illustrates the typical observed XRD profile for 50:50 Al2 O3 /LaPO4 ceramic composite. It is seen that Al2 O3 peaks undergo a peak shift and broadening indicating lattice/phase straining possibly due to refinement of the alumina at higher order LaPO4 content. Typical XRD profile of machined powder Al2 O3 /LaPO4 (50:50) ceramic composite is shown in Fig. 5(f). It can be seen that machined powder of 50:50 composite is richer in Al2 O3 content compared to 70:30 machined powder. This can be attributed to possible reduced order sinterability with higher order LaPO4 content. The relatively softer LaPO4 could have fallen off. Referring to the XRD intensity profile of the machined powder, it is seen that peaks pertaining to 70:30 composites exhibits more relaxation compared to plain alumina or 50:50 proportion. This attributes to the better sintering status with 70:30 proportion. Typical XRD intensity profile of sintered LaPO4 specimen and machined LaPO4 powder chips are illustrated in Fig. 5(g) and (h). As in the earlier cases, XRD profile of machined powder chips indicate induced material stressing with mixed mode, attributable to the condition of the sintered LaPO4 .
3.6.
Acoustic emission analysis
Response of the ceramic composites to ultrasonic drilling has been assessed by monitoring the AE signal emitted from the work piece. AE is relatively a weak elastic stress wave emitted by a material under stress and thus it is a reliable indicator of the status of the material. The signal usually of higher frequency and smaller amplitude is picked up by a broad band piezoelectric sensor and amplified further for signal analysis. Typical power spectra of the signals monitored are illustrated in this section. Power spectrum of AE signal monitored during the entry of the tool during ultrasonic drilling of alumina is illustrated in Fig. 6(a). The signal is of mixed mode with dominant peaks over 50–100 KHz and 250–300 KHz frequency range. During tool entry, the work material experiences fluffing associated with continuous erosion and cracking of material. The observed higher frequency peaks can be attributed to cracking of the surface material during entry. With depth of penetration, the erosion will be steady as reflected in relatively reduced amplitude of peaks over 50–100 KHz. With progressive depth of penetration a mild increase in the power of AE signal monitored can be seen. As depth of penetration increases, the effective thickness of the work piece to be drilled, reduces resulting in the observed rise in the AE signal power. The signal monitored at the exit exhibits higher power owing to exit fluffing. In drilling of LaPO4 , owing to its lower hardness, the power of AE signal monitored drops down, as indicated by relatively lower power emitted (Fig. 6(b)). With depth of penetration there is a steady drop in the power as indicated by reduction in power of the signal monitored. At the exit, a
Fig. 7 – Variation of AE rms for solid tool.
marginal rise in power can be seen in the signal containing low frequency peaks. Drilling of Al2 O3 /LaPO4 (70:30) ceramic composite exhibits relatively enhanced performance as indicated by the absence of any burst mode (high frequency peaks). The reinforcement of Al2 O3 and LaPO4 could have arrested the cracking tendency facilitating better machining. With increase in depth of penetration, the machining rate is more or less maintained as indicated by a marginal reduction in peak amplitude (Fig. 6(c)). At exit, the signal monitored does not contain any high frequency peaks indicating absence of any cracking/fluffing tendency. The nature of AE signal monitored in the case of 70:30 ceramic composite indicates improved and sustained machining. In the case of Al2 O3 /LaPO4 composite of 50:50 composition, relatively low power AE signal (Fig. 6(d)) was monitored, attributable to the reduced hardness of the composite and reduced order of sinterability with higher LaPO4 content. Typical observed variation of rms values of the acoustic emission signal monitored during ultrasonic drilling of Al2 O3 /LaPO4 composite is illustrated in Fig. 7. It is seen that alumina releases high power AE signal at entry and exit as indicated by the higher magnitude of rms values. Drilling of LaPO4 results in more or less steady emission as illustrated by only a marginal variation in AE rms value. This is supported by the observed marginal variation in hardness with the indentation load. 70:30 Al2 O3 /LaPO4 ceramic composite exhibits a relatively reduced power of emission at the tool entry, followed by a marginal rise in power of emission with depth of penetration of the tool. This is attributed to the better quality of sintering and dispersion strengthening associated with 70:30 composition. With 50:50 composition, a wide variation of AE rms values can be seen, attributable to poor sintering quality of the composite.
4.
Conclusions
From the observation on ultrasonic drilling of Al2 O3 /LaPO4 ceramic composites, the following major conclusions are drawn.
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• The microhardness decreased gradually with increase in LaPO4 content up to 30% and beyond this, hardness dropped appreciably indicating the limiting value of the addition for structural compatibility. • The hardness of the Al2 O3 /LaPO4 composites showed a progressive increase with indenter load when LaPO4 content was low, but with higher LaPO4 content, only a marginal variation in hardness was observed, indicating poor sinterability. • As the LaPO4 content increased, machinability also increases. However with higher order LaPO4 content a reduction in MRR sets in, indicating the reduced applicability of higher proportion to ultrasonic drilling. • Monitoring of acoustic emission indicates the response of the composite material to ultrasonic drilling in terms of power spectrum and rms value. • X-ray diffraction indicated the presence of compressive residual stresses due to sintering of composites and relaxation of the stresses induced in the machined composites. • In all cases, with hollow tool, appreciable enhancement in MRR is observed. The study indicates that the addition of LaPO4 up to 30% can be attempted for enhanced sinterability and machinability of the composites.
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Acknowledgement The authors would like to thank the Ceramics Division, Regional Research Laboratory, Thiruvananthapuram, India, for providing the specimens for this study.
references
Davis, J.B., Marshall, D.B., Housley, R.M., Morgan, P.E.D., 1998. Machinable ceramics containing rare-earth phosphates. Journal of American Ceramic Society 81 (8), 2169–2175. Luo, Y., Li, S., Pan, W., Chen, J., Wang, R., 2004. Machinable and mechanical properties of sintered Al2 O3 –Ti3 SiC2 composites. Journal of Materials Science 39, 3137–3140. Rozenberg, L.D., Kazantsev, V.F., Makarov, L.O., Yakhimovich, D.F., 1964. Ultrasonic Cutting. Consultants Bureau, New York. Wang, R., Pan, W., Chen, J., Fang, M., Meng, J., 2002. Effect of LaPO4 content on the microstructure and machinability of Al2 O3 /LaPO4 composites. Materials Letters 57, 822–827. Wang, R., Pan, W., Chen, J., Jiang, M., Luo, Y., Fang, M., 2003a. Properties and microstructure of machinable Al2 O3 /LaPO4 ceramic composites. Ceramics International 29, 19–25. Wang, R., Pan, W., Chen, J., Fang, M., Jiang, M., Cao, Z., 2003b. Microstructure and mechanical properties of machinable Al2 O3 /LaPO4 composites by hot pressing. Ceramic International 29, 83–89.