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Microhardness studies on triglycine sulphate and diglycine sulphate single crystals
Surface Technology, 22 (1984) 381 - 385
381
MICROHARDNESS STUDIES ON TRIGLYCINE SULPHATE AND DIGLYCINE SULPHATE SINGLE CRYSTALS
G. R. PANDYA, D. D. VYAS and C. F. DESAI
Physics Department, Faculty of Science, Maharaja Sayajirao University of Baroda, Baroda 390002 (India) (Received February 15, 1984)
Summary The (010) cleavage planes of triglycine sulphate (TGS) and anhydrous diglycine sulphate (DGS) crystals were indented using a diamond pyramidal indenter. The variation in the microhardness with load is reported, and the features accompanying the indentation marks on these crystals are discussed. The hardness of TGS is 215 kgf mm -2 while that of DGS is 78 kgf mm -2.
1. Introduction Although triglycine sulphate (TGS) crystals have generated much interest and are now quite well studied, the work carried out so far has mainly been concerned with growth, domain studies and ferroelectric properties. The TGS slip system has been reported by Konstantinova [ 1]. In contrast, anhydrous diglycine sulphate (DGS) is a non-ferroelectric crystalline phase of glycine sulphate, which has been studied little. Except for the growth and crystallography of DGS, very little information is available about its physical properties [2]. No report on the microhardness of DGS has been found in the literature. The present authors have reported the deformation characteristics and active slip systems of DGS [3]. In this paper, the microhardness of these crystals, including the features accompanying the indentation marks, are reported.
2. Experimental details Transparent single crystals of TGS and DGS were obtained from aqueous solutions of glycine and sulphuric acid by evaporation at a constant temperature (28 °C). The pH of the solution for TGS was maintained at 1.5 while that for DGS was fixed at 1.2 [4]. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
382
Both as-gown and thermally treated crystals were used in the present study. In the thermal treatment some crystals were annealed at 90 °C for 12 h and then slowly cooled while others were quenched to room temperature. The (010) cleavage planes of both the TGS and the DGS crystals were used. Only freshly cleaved samples obtained from the as-grown and thermally treated crystals were used for the hardness measurements. The hardness indentations were made by means of a Vickers' diamond pyramidal indenter (on the hardness testing system of a Vickers' projection microscope) with a range of loads up to 100 gf. The indenter is a squarebased pyramid, the opposite faces of which make an angle of 136 ° with each other. Care was taken to m o u n t the specimen with its surface normal to the indenter. The indentation was carried out keeping in mind the necessary precautions suggested by the supplier.
3. Results
3.I. Indentation features At low loads, less than 1 gf, inconsistent results were obtained, mainly as a result of the limited sensitivity of the instrument. Also at higher loads, above 50 gf, the hardness values were n o t consistent because of a large number of deformation cracks resulting in distorted indentation marks. However, the range of loads giving consistent results (1 - 50 gf) includes the essential range of interest in the microhardness region and the results discussed below are confined to the microhardness corresponding to this range of loads. For both TGS and DGS crystals, the indentation marks were perfect squares. However, there were cracks produced which emerged from the comers of the indentation marks. The cracks produced at the comers were f o u n d to be present irrespective of the orientation of the indenter. These prominent cracks extending along the diagonal of the indentation marks
Fig. 1. Indentation marks on the (010) plane of DGS crystals. (Magnification, 225×.) Fig. 2. Indentation marks on the (010) plane of TGS crystals. (Magnification, 225×.)
383 were produced at almost all the loads for the DGS crystals (Fig. 1). In TGS crystals, cracks were usually produced when the crystals were indented with loads above 2 gf. Moreover, the cracks produced in the TGS crystals were n o t always found to be straight and to extend along the diagonal as for DGS crystals. This is illustrated in Fig. 2. The irregular geometry of the cracks is n o t e w o r t h y : in particular, the cracks m a y even be curved and may join together to form a loop, as can be seen in Fig. 2. Thus, while the fracture of DGS crystals has been found to follow crystallographic directions closely [3], the high load response of the TGS crystals results in irregular chipping and fragmenting. The above feature of the cracks on the TGS crystals reflects this fact at moderate loads, indicating the high brittleness of TGS compared with that o f DGS.
3.2. Hardness and its variation with load
To study the variation in hardness with load, indentations were made under various loads at room temperature on the (010) planes of annealed crystals of TGS and DGS. The indentations on as-grown cleaved crystals did n o t always give consistent results, particularly at low loads. This may be due to the high tendency of the crystals to trap inclusions of the m o t h e r liquor during their growth from solution [5]. In the heat treatment of the crystals, the liquid in the inclusions is found to escape. Apart from this, and more significantly, the annealing o f the crystals redistributes the imperfections and makes them stable. Hence annealed crystals gave more consistent results in the microhardness measurements. Quenching provides thermal shock resulting in deformation and cracks. Hence the microhardness measurements on such specimens showed an erratic behaviour, implying that annealed crystals were more reliable for the present study. Since many crystals have been found to possess hardness anisotropy, the orientation dependence of the hardness, if any, was checked for these crystals. However, it was observed that variation in the orientation of the indenter with respect to the crystal did n o t produce any significant change in the hardness values. Figures 3 and 4 show plots of the hardness H v against the applied load P for annealed DGS and TGS crystals respectively. The plots clearly indicate that the hardness varies with load in a complex manner. It is observed that at low loads the hardness in both cases increases with increasing load. After a certain peak value has been attained, a sharp decrease is observed. With further increases in the load, the hardness increases slightly and then after a slight drop it attains a constant value. The peak value for DGS is 86 kgf mm -2 while that for TGS is 215 kgf mm -2. DGS attains a constant hardness value of 78 kgf mm -2 at loads greater than 10 gf. However, TGS achieves a constant value of 240 kgf mm -2 at loads greater than 25 gf. It can be seen from the general nature of the plots that the load dependence of the hardness o f TGS crystals is much more complex than that of the DGS crystals. It can also be seen that TGS has
384
> ~:
4o
2O
'
.
.
.
.
.
'
24
~.6
P (gf)
Fig. 3. A plot of hardness H v
vs.
applied load P for annealed DGS crystals.
250
I
2oo
E E 15c > !.o(
5c
'
{
'
i 16
~
2t4
~
~ SZ
h
,~o
P (gf)
Fig. 4. A plot o f hardness H v
vs.
applied load P for annealed TGS crystals.
a b o u t a threefold higher hardness than DGS. This agrees with the greater tendency of TGS to fracture compared with DGS, since in general hard materials are also likely to be more brittle because of limited plasticity. 4. Conclusions
(i) Both TGS and DGS crystals are hard materials; however, TGS is much harder than DGS. The hardness of TGS is 215 kgf m m -2 while t h a t o f DGS is 78 kgf mm -2. (ii) The crack and fracture characteristics of TGS and DGS crystals indicate that TGS is more brittle than DGS.
385
Acknowledgments The authors are grateful to Professor S. K. Shah and to Dr. V. P. Bhatt for their keen interest in this work.
References 1 V.P. Konstantinova, Soy. Phys. - - Crystallogr., 7 (5) (1963) 605. 2 P. W. Whipps, R. S. Cosier and K. L. Bye, J. Mater. Sci., 7 (1972) 1476. 3 G. R. Pandya, D. D. Vyas and C. F. Desai, Soy. Phys. - - Crystallogr., 2 7 (4) (1982) 426. 4 G. R. Pandya and D. D. Vyas, J. Cryst. G r o w t h , 5 0 (1980) 870. 5 G. R. Pandya and D. D. Vyas, Cryst. Res. Technol., 16 (1981) 319.
381
MICROHARDNESS STUDIES ON TRIGLYCINE SULPHATE AND DIGLYCINE SULPHATE SINGLE CRYSTALS
G. R. PANDYA, D. D. VYAS and C. F. DESAI
Physics Department, Faculty of Science, Maharaja Sayajirao University of Baroda, Baroda 390002 (India) (Received February 15, 1984)
Summary The (010) cleavage planes of triglycine sulphate (TGS) and anhydrous diglycine sulphate (DGS) crystals were indented using a diamond pyramidal indenter. The variation in the microhardness with load is reported, and the features accompanying the indentation marks on these crystals are discussed. The hardness of TGS is 215 kgf mm -2 while that of DGS is 78 kgf mm -2.
1. Introduction Although triglycine sulphate (TGS) crystals have generated much interest and are now quite well studied, the work carried out so far has mainly been concerned with growth, domain studies and ferroelectric properties. The TGS slip system has been reported by Konstantinova [ 1]. In contrast, anhydrous diglycine sulphate (DGS) is a non-ferroelectric crystalline phase of glycine sulphate, which has been studied little. Except for the growth and crystallography of DGS, very little information is available about its physical properties [2]. No report on the microhardness of DGS has been found in the literature. The present authors have reported the deformation characteristics and active slip systems of DGS [3]. In this paper, the microhardness of these crystals, including the features accompanying the indentation marks, are reported.
2. Experimental details Transparent single crystals of TGS and DGS were obtained from aqueous solutions of glycine and sulphuric acid by evaporation at a constant temperature (28 °C). The pH of the solution for TGS was maintained at 1.5 while that for DGS was fixed at 1.2 [4]. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
382
Both as-gown and thermally treated crystals were used in the present study. In the thermal treatment some crystals were annealed at 90 °C for 12 h and then slowly cooled while others were quenched to room temperature. The (010) cleavage planes of both the TGS and the DGS crystals were used. Only freshly cleaved samples obtained from the as-grown and thermally treated crystals were used for the hardness measurements. The hardness indentations were made by means of a Vickers' diamond pyramidal indenter (on the hardness testing system of a Vickers' projection microscope) with a range of loads up to 100 gf. The indenter is a squarebased pyramid, the opposite faces of which make an angle of 136 ° with each other. Care was taken to m o u n t the specimen with its surface normal to the indenter. The indentation was carried out keeping in mind the necessary precautions suggested by the supplier.
3. Results
3.I. Indentation features At low loads, less than 1 gf, inconsistent results were obtained, mainly as a result of the limited sensitivity of the instrument. Also at higher loads, above 50 gf, the hardness values were n o t consistent because of a large number of deformation cracks resulting in distorted indentation marks. However, the range of loads giving consistent results (1 - 50 gf) includes the essential range of interest in the microhardness region and the results discussed below are confined to the microhardness corresponding to this range of loads. For both TGS and DGS crystals, the indentation marks were perfect squares. However, there were cracks produced which emerged from the comers of the indentation marks. The cracks produced at the comers were f o u n d to be present irrespective of the orientation of the indenter. These prominent cracks extending along the diagonal of the indentation marks
Fig. 1. Indentation marks on the (010) plane of DGS crystals. (Magnification, 225×.) Fig. 2. Indentation marks on the (010) plane of TGS crystals. (Magnification, 225×.)
383 were produced at almost all the loads for the DGS crystals (Fig. 1). In TGS crystals, cracks were usually produced when the crystals were indented with loads above 2 gf. Moreover, the cracks produced in the TGS crystals were n o t always found to be straight and to extend along the diagonal as for DGS crystals. This is illustrated in Fig. 2. The irregular geometry of the cracks is n o t e w o r t h y : in particular, the cracks m a y even be curved and may join together to form a loop, as can be seen in Fig. 2. Thus, while the fracture of DGS crystals has been found to follow crystallographic directions closely [3], the high load response of the TGS crystals results in irregular chipping and fragmenting. The above feature of the cracks on the TGS crystals reflects this fact at moderate loads, indicating the high brittleness of TGS compared with that o f DGS.
3.2. Hardness and its variation with load
To study the variation in hardness with load, indentations were made under various loads at room temperature on the (010) planes of annealed crystals of TGS and DGS. The indentations on as-grown cleaved crystals did n o t always give consistent results, particularly at low loads. This may be due to the high tendency of the crystals to trap inclusions of the m o t h e r liquor during their growth from solution [5]. In the heat treatment of the crystals, the liquid in the inclusions is found to escape. Apart from this, and more significantly, the annealing o f the crystals redistributes the imperfections and makes them stable. Hence annealed crystals gave more consistent results in the microhardness measurements. Quenching provides thermal shock resulting in deformation and cracks. Hence the microhardness measurements on such specimens showed an erratic behaviour, implying that annealed crystals were more reliable for the present study. Since many crystals have been found to possess hardness anisotropy, the orientation dependence of the hardness, if any, was checked for these crystals. However, it was observed that variation in the orientation of the indenter with respect to the crystal did n o t produce any significant change in the hardness values. Figures 3 and 4 show plots of the hardness H v against the applied load P for annealed DGS and TGS crystals respectively. The plots clearly indicate that the hardness varies with load in a complex manner. It is observed that at low loads the hardness in both cases increases with increasing load. After a certain peak value has been attained, a sharp decrease is observed. With further increases in the load, the hardness increases slightly and then after a slight drop it attains a constant value. The peak value for DGS is 86 kgf mm -2 while that for TGS is 215 kgf mm -2. DGS attains a constant hardness value of 78 kgf mm -2 at loads greater than 10 gf. However, TGS achieves a constant value of 240 kgf mm -2 at loads greater than 25 gf. It can be seen from the general nature of the plots that the load dependence of the hardness o f TGS crystals is much more complex than that of the DGS crystals. It can also be seen that TGS has
384
> ~:
4o
2O
'
.
.
.
.
.
'
24
~.6
P (gf)
Fig. 3. A plot of hardness H v
vs.
applied load P for annealed DGS crystals.
250
I
2oo
E E 15c > !.o(
5c
'
{
'
i 16
~
2t4
~
~ SZ
h
,~o
P (gf)
Fig. 4. A plot o f hardness H v
vs.
applied load P for annealed TGS crystals.
a b o u t a threefold higher hardness than DGS. This agrees with the greater tendency of TGS to fracture compared with DGS, since in general hard materials are also likely to be more brittle because of limited plasticity. 4. Conclusions
(i) Both TGS and DGS crystals are hard materials; however, TGS is much harder than DGS. The hardness of TGS is 215 kgf m m -2 while t h a t o f DGS is 78 kgf mm -2. (ii) The crack and fracture characteristics of TGS and DGS crystals indicate that TGS is more brittle than DGS.
385
Acknowledgments The authors are grateful to Professor S. K. Shah and to Dr. V. P. Bhatt for their keen interest in this work.
References 1 V.P. Konstantinova, Soy. Phys. - - Crystallogr., 7 (5) (1963) 605. 2 P. W. Whipps, R. S. Cosier and K. L. Bye, J. Mater. Sci., 7 (1972) 1476. 3 G. R. Pandya, D. D. Vyas and C. F. Desai, Soy. Phys. - - Crystallogr., 2 7 (4) (1982) 426. 4 G. R. Pandya and D. D. Vyas, J. Cryst. G r o w t h , 5 0 (1980) 870. 5 G. R. Pandya and D. D. Vyas, Cryst. Res. Technol., 16 (1981) 319.