Joshi, D R

Shah, B S

Vaidya, Nimisha

IACS Institutional Repository

Indian J. Phys. 70 A (5), 641-644 (1996)NOTEI J P  Aan international journalKnoop hardness studies on benzoic acid crystalsN i m i s h a  V a i d y a  a n d  B  S  S h a h  Department of Physics, Saurashlra University,Rajkot-360 005, Saurashtra, India andD  R  J o s h iDepartment of Applied Physics. M S University of Baroda, Baroda-390 002, IndiaRe( eivcd 27 (h tober 1995, uct epted 12 February 1996Abstract : Knoop microhardness studies were earned out on ben/oic acid single ciysials The hardness versus load plot shows two peaks at 3 gin and b gm loads having hardness value ot 6 2 kg/inm- each. This corresponds to the first peak at 20 gm load observed with pyramidal hardness studies The peak was due to the slip plane in the ( 100) |0 I0 | diiection The present observation suggests that the ( 100) [0 10 ] dislocations split into partialsKeywords : Knoop indentation, nncrohardness, ben/oic acid PACTS Nos. : 8 1 40 Np, 6 1 72 Hh, 62 20 FeMolecular solids arc divided into two broad categories, the plastic and the non-plastic. Plastic crystals have nearly spherical molecules forming a cubic or a highly symmetric crystal structure [1]. The non-plastic crystals consist in general of non-spherically symmetric molecules which pack together as closely as the geometry allows to dciine a crystallographic structure. These solids are difficult to deform plastically at normal ambient temperatures and pressures in the bulk. For these materials, the study of plastic deformation produced by point loading is of practical importance as the results can be related to mechanical properties.Single crystals of benzoic acid were grown from the melt by the Bridgemann method. The starting material was column chromatographed, twice vacuum sublimed and /onc-rcfined. The material was transferred to the crystal growth tubes without exposure to ihe atmosphere [2]. The crystals were cleaved in the usual manner using a sharp blade.©  1 9 9 6 1 A C S6 42 Nimisha Vaidya, B S Shah and D R JoshiSmooth (001) cleavages were selected after optical examination. The crystals were indented on a Carl Zeiss NU2 Universal Research Microscope.Indents were made w ith a Knoop Indenter. Loads (P) varied from 1.2S gm to 17.5 gm in small steps. A num ber o f indents were made at each load. The long diagonal length was used in calculating the Knoop Hardness Number (KHN) using the formulaKHN = 14228.8 x  P / d 1, (1)where P is the applied load in gm and d  the m ean long diagonal length in pm . The indentation time o f 10 s was kept constant. The crystal size was much larger than the indentation sizes, thus eliminating the boundary effects on the results. The distance between the indents was five times the size o f  the largest indentation mark. The crystal thickness was relatively more such that the indenter did not sense the lower surface [3]. A number of crystals were studied.Figure 1 shows the hardness variations with load for benzoic acid. The plot exhibits two peaks very close to one another and having the same hardness value at loads around 3 gm and 6 gm.BENZOIC ACIO KNOOP HARDNESS VS LOADFigure 1. Hardness variation with load.There are a few papers on the pyramidal hardness o f some organic and inorganic m olecular solids [4-6]. In all these studies one observes that the load variation has been studied using the standard set o f weights available in the weight box. Some detailed studies have been done using weights having small differences [7]. These studies show that the plots o f  hardness variation with load is just not a curve which decreases or increases at lowKnoop hardness studies on benzoic acid crystals 643l o a d s  a n d  t h e n  r e m a i n s  s t e a d y  b u t  e x h i b i t s  t w o  a n d  s o m e t i m e s  t h r e e  p e a k s .  T h e s e  p e a k s  h a v e  b e e n  a c c o u n t e d  f o r  s l i p  t a k i n g  p l a c e  o n  d i f f e r e n t  p l a n e s  s l i p p i n g  i n  a  c o m m o n  d i r e c t i o n .T h e r e  a r e  n o  s t u d i e s  o n  t h e  h a r d n e s s  v a r i a t i o n  w i t h  l o a d  u s i n g  a  K n o o p  I n d e n t e r  i n  o r g a n i c  m o l e c u l a r  s o l i d s  t o  t h e  b e s t  o f  t h e  a u t h o r s  k n o w l e d g e .T h e  h a r d n e s s  o f  a  m a t e r i a l  d e p e n d s  o n  i t s  p l a s t i c  a n d  c l a s t i c  p r o p e r t i e s .  F o r  K n o o p  i n d e n t a t i o n ,  e l a s t i c  r e c o v e r y  t a k e s  p l a c e  i n  t h e  s h o r t  d i a g o n a l  o n l y  a n d  t h e  l o n g  d i a g o n a l  d o e s  n o t  c h a n g e  w h e n  t h e  l o a d  i s  r e m o v e d .  W h e n  a  l o a d e d  i n d e n t e r  p e n e t r a t e s  a  m a t e r i a l ,  t h e  d e p t h  o f  p e n e t r a t i o n  i n c r e a s e s  u n t i l  t h e  c o n d i t i o n  o f  t h e  l a t t i c e  i m m e d i a t e l y  b e l o w  t h e  i n d e n t e r  h a s  t h e  s a m e  c h a r a c t e r i s t i c s  a s  t h a t  o f  a  s p e c i m e n  i n  w h i c h  s a t u r a t i o n  v a l u e  o f  t h e  c o m p r e s s i v e  s t r a i n  h a s  b e e n  a t t a i n e d .  T h e  s t r a i n  b e c o m e s  p r o g r e s s i v e l y  s m a l l e r  w i t h  i n c r e a s i n g  d i s t a n c e  f r o m  t h e  c e n t e r  o f  t h e  m a t e r i a l  t o  a  d i s t a n c e  e q u a l  t o  t h e  l e n g t h  o f  t h e  l o n g  d i a g o n a l .P y r a m i d a l  i n d e n t a t i o n  s t u d i e s  o n  t h e  ( 0 0 1 )  c l e a v a g e s  o f  b e n z o i c  a c i d  s i n g l e  c r y s t a l s  s h o w  t w o  p e a k s  i n  t h e  p l o t  o f  t h e  h a r d n e s s  w i t h  l o a d  a t  2 0  g m  a n d  8 0  g m  w i t h  t h e  V i c k e r ’ s  H a r d n e s s  v a l u e s  b e i n g  6 . 0 2  k g / m m 2 a n d  5 . 7 0  k g / m m 2 r e s p e c t i v e l y  [ 8 ] .  T h e s e  t w o  p e a k s  h a v e  b e e n  a s s i g n e d  d u e  t o  t h e  d i s l o c a t i o n  o f  t h e  t y p e  ( 1 0 0 )  1 0 1 0 )  a n d  ( 0 1 0 )  1 1 0 0 ] .K n o o p  i n d e n t a t i o n  s t u d i e s  c o u l d  b e  c a r r i e d  o u t  o n l y  f o r  l o a d  u p t o  2 0  g m  a t  s m a l l  i n t e r v a l s .  S t u d i e s  a t  h i g h e r  l o a d s  c a u s e d  t h e  l e n g t h  o f  t h e  l o n g  d i a g o n a l  t o  b e c o m e  l a r g e  a n d  o u t  o f  t h e  m e a s u r i n g  v i e w .  W h e n  t h e  K n o o p  h a r d n e s s  v a l u e s  a r e  p l o t t e d  a g a i n s t  t h e s e  l o a d s ,  t w o  p e a k s  v e r y  c l o s e  t o  e a c h  o t h e r  a t  3  g m  a n d  6  g m  l o a d  a r e  o b s e r v e d  w i t h  a  h a r d n e s s  v a lu e  o f  6 . 2  k g / m m 2 f o r  e a c h  p e a k .T h e  f i r s t  p e a k  i n  t h e  p y r a m i d a l  h a r d n e s s  s t u d ie s  h a s  b e e n  a s s i g n e d  t o  d i s l o c a t i o n s  o f  t h e  t y p e  ( 1 0 0 )  [ 0 1 0 ]  w h i c h  o c c u r s  a t  2 0  g m  l o a d  h a v i n g  a  h a r d n e s s  v a l u e  o f  6 . 2  k g / m m 2 . A s  t h e  h a r d n e s s  v a l u e  i s  t h e  s a m e  a s  t h a t  i n  t h e  p y r a m i d a l  h a r d n e s s  s t u d ie s  a n d  t h e  p e a k s  o c c u r  a t  l o w e r  l o a d s  t h a n  t h a t  o f  t h e  p y r a m i d a l  h a r d n e s s  c u r v e  o f  h a r d n e s s  a g a i n s t  l o a d ,  i t  i s  c o n j e c t u r e d  t h a t  t h e  d i s l o c a t i o n  ( 1 0 0 )  [ 0 1 0 ]  s p l i t s  i n t o  p a r t i a l s  a s  h a s  b e e n  s a id  t o  o c c u r  i n  a n t h r a c e n e  [ 9 ] .T h e  c o n c l u s i o n s  a r e  :1 . T h e  K n o o p  h a r d n e s s  v a r i a t i o n  w i t h  l o a d  g i v e s  r i s e  t o  t w o  p e a k s  v e r y  c l o s e  a t  3  g ma n d  6  g m  l o a d  w i t h  a  h a r d n e s s  v a l u e  o f  6 . 2  k g / m m 2 .2 .  T h e s e  t w o  p e a k s  a r e  c a u s e d  d u e  t o  t h e  s p l i t t i n g  o f  t h e  ( 1 0 0 )  1 0 1 0 ]  d i s l o c a t i o n  i n t op a r t i a l s .Referencestl] J Timmermans Bull. Soc. Chint. Beige. 44 17 (1935)[2] J N Sherwood Fractional Crystallization Vol. 2 ed. Zief p 175 (1969)644 N im isha Vaidyci, B S Shah and D R Joshi|3| B W Mott Micro-Indentation Hardness Testing (London : Buttcrworths) (1956)[41 F M Hampton, R M Hooper, B S Shah, J N Sherwood, J Di-Persio and B Escaig Phil. Max 29743(1974)[5] P J Halfpenny. K J Roberts and J N Sherwood J Mater Set 19 1629 (1984)[6] M J Joshi and B S Shah Cryst. Res. Technoi. 19 1107 (1984)[71 R K Murwaha and B S Shah Cryst Res. Technoi. 23 K63 (1988)[8| R K Murwaha and B S Shah Cryst Res. Technoi 26 491 (1991)[9] J O Williams St i. Pro}>. Oxf. 64 247 (1977)

Knoop Hardness Studies on Benzoic Acid Crystals

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Knoop Hardness Studies on Benzoic Acid Crystals

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