MECHANICAL PROPERTIES OF THIOUREA BASED ORGANIC AND SEMIORGANIC SINGLE CRYSTALS

Transcription

1 153 CHAPTER 7 MECHANICAL PROPERTIES OF THIOUREA BASED ORGANIC AND SEMIORGANIC SINGLE CRYSTALS 7.1 INTRODUCTION Hardness of a material is the resistance it offers to indentation by a much harder body. It may be termed as a measure of the resistance against lattice destruction or the destruction or the resistance offered to permanent deformation or damage. The term "hardness" means in different ways. It is the resistance to penetration to a metallurgist, resistance to wear to a lubrication engineer, resistance to scratching to a mineralogist and resistance to cutting to a machinist. All these are related to the plastic flow stress of the material. The hardness properties are basically related to the crystal structure of the materials. Microhardness study on the crystals brings out an understanding of the plasticity of the crystal. Hardness is a technique, in which a crystal is subjected to a relatively high pressure within a localized area. By suitable choice of indenter material and relatively simple equipment construction, hardness tests can be easily carried out on all crystalline materials under various conditions of temperature and pressure. Deformation is local, so that a number of trials can be made on a single specimen of small dimension and can be reproduced by maintaining the specimen indenter orientation relationship. Specimen of flat relatively smooth surface is required.

2 ANALYSIS METHODS Methods of Hardness Test Hardness measurement can be carried out by various methods. They are classified as follows, 1. Static indentation test 2. Dynamic indentation test 3. Scratch test 4. Rebound test 5. Abrasion test Static Indentation Test The most popular and simplest form is the static indentation test wherein the specific geometry is pressed into the surface of a test specimen under a known load. The indenter may be ball or diamond cone or diamond pyramid. Upon removal of the indenter, a permanent impression is retained in the specimen. The hardness is calculated from the area or depth of indentation produced. The variables are the type of the indenter or load. The indenter is made up of a very hard material to prevent its deformation by the test piece so that it can cover materials over a wide range of hardness. For this reason either a hardness steal sphere or a diamond pyramid or cone is employed. A pyramid also has advantage that geometrically similar impressions are obtained at different loads. So naturally a pyramid indenter is preferred. In this static indentation test the indenter is pressed perpendicularly in the surface of the sample by means of an applied load. Then by measuring the

3 155 cross sectional area or the depth of the indentation and knowing the applied load an empirical hardness number may be calculated. This procedure followed by Brinell, Meyer, Knoop and Rockwell test Dynamic Indentation Test In the dynamic indentation test, a ball or a cone (or a number of small spheres) is allowed to fall from a definite height and the hardness number is obtained from the dimensions of the indentation and the energy of impact Scratch Test The scratch test can be classified into two types: a. Comparison test in which one material is said to be harder than another if the second material is scratched by the first. b. A scratch test with a diamond indenter on the surface at a steady rate and under a definite load. The hardness number is expressed in terms of the width of the groove formed Rebound and Abrasion Test In the rebound test an object of standard mass and dimensions is bounced from the test surface and the height of rebound is taken as the measure of hardness. In abrasion test, a specimen is loaded against a rotating disk and the rate of wear is taken as a measure of hardness.

4 156 Figure 7.1 Types of hardness test Brinell Hardness Test The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation. The Brinell Hardness Number is calculated by: BHN = [ ] (7.1) F : applied load, kg D: Diameter of the ball indenter, mm d : mean diameter of impression, mm

5 157 Figure 7.2 Scheme of Brinell hardness test The diameter of the impression is the average of two readings at right angles and the use of a Brinell hardness number table can simplify the determination of the Brinell hardness. A well structured Brinell hardness number reveals the test conditions, and looks like this, "75 HB 10/500/30" which means that a Brinell Hardness of 75 was obtained using a 10mm diameter hardened steel with a 500 kilogram load applied for a period of 30 seconds. On tests of extremely hard metals a tungsten carbide ball is substituted for the steel ball. Compared to the other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures and any irregularities in the uniformity of the material. This method is the best for achieving the bulk or macro-hardness of a material, particularly those materials with heterogeneous structures Rockwell Hardness Test The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load F0 (Figure 7.3.A)

6 158 usually 10 kgf. When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter, is set to a datum position. While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration (Figure 7.3.B). When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Figure7.3.C). The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number. HR = E e (7.2) Figure 7.3 scheme of Rockwell hardness test F0 = preliminary minor load in kgf F1 = additional major load in kgf F = total load in kgf e = permanent increase in depth of penetration due to major load F1 measured in units of mm E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter

7 159 HR = Rockwell hardness number D = diameter of steel ball Advantages of the Rockwell hardness method include the direct Rockwell hardness number readout and rapid testing time. Disadvantages include many arbitrary non-related scales and possible effects from the specimen support anvil (try putting a cigarette paper under a test block and take note of the effect on the hardness reading, Vickers and Brinell methods don't suffer from this effect) The Scleroscope Hardness Test The Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a glass tube under the force of its own weight from a fixed height, onto the test specimen. The height of the rebound travel of the hammer is measured on a graduated scale. The scale of the rebound is arbitrarily chosen and consists on Shore units, divided into 100 parts, which represent the average rebound from pure hardened high-carbon steel. The scale is continued higher than 100 to include metals having greater hardness. The Shore Scleroscope measures hardness in terms of the elasticity of the material and the hardness number depends on the height to which the hammer rebounds, the harder the material, the higher the rebound The Durometer The Durometer is a popular instrument for measuring the indentation hardness of rubber and rubber-like materials. The operation of the tester is quite simple. The material is subjected to a definite pressure applied by a calibrated spring to an indenter that is either a cone or sphere and an indicating device measures the depth of indentation.

8 Moh's Hardness Scale The Moh's hardness scale for minerals has been used since It simply consists of 10 minerals arranged in order from 1 to 10. Diamond is rated as the hardest and is indexed as 10; talc as the softest with index number 1. Each mineral in the scale will scratch all those below it as follows: The steps are not of equal value and the difference in hardness between 9 and 10 is much greater than between 1 and 2. The hardness is determined by finding which of the standard minerals the test material will scratch or not scratch; the hardness will lie between two points on the scale - the first point being the mineral which is scratched and the next point being the mineral which is not scratched. Some examples of the hardness of common metals in the Moh's scale are copper between 2 and 3 and tool steel between 7 and 8. This is a simple test, but is not exactly quantitative and the standards are purely arbitrary numbers. Table 7.1 Moh's hardness scale for minerals Diamond 10 Corundum 9 Topaz 8 Quartz 7 Orthoclase (Feldspar) 6 Aptite 5 Fluorite 4 Calcite 3 Gypsum 2 Talc 1

9 Vickers Hardness Test The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation. Figure 7.4 Scheme of Vickers hardness test P = Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness HV= [2P sin (136º/2)] / d 2 or HV = (P/d 2 ) (7.3)

10 162 Figure 7.5 Pyramidal form of diamond indenter Figure 7.6 photography of indentation When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force. Several different loading settings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness testing methods. The advantages of the Vickers hardness test are that extremely accurate readings can be taken,

11 163 and just one type of indenter is used for all types of metals and surface treatments. Although thoroughly adaptable and very precise for testing the softest and hardest of materials, under varying loads, the Vickers machine is a floor standing unit that is more expensive than the Brinell or Rockwell machines. There is now a trend towards reporting Vickers hardness in SI units (MPa or GPa) particularly in academic papers. Unfortunately, this can cause confusion. Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm 2 ). Rigorous application of SI is a problem. Most Vickers hardness testing machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and messy to engineers and technicians. To convert Vickers hardness number the force applied needs converting from kgf to newtons and the area needs converting form mm 2 to m 2 to give results in Pascal using the formula above. To convert HV to MPa multiply by To convert HV to GPa multiply by Work Hardening Coefficient and Vickers Hardness Measurement Microhardness of a crystal is its capacity to resist indentation. Physically hardness is the resistance offered by a material to the localized deformations caused by scratching or by indentations (Mukerji 1999 and Subhadra 2000). Microhardness and anisotropy study Vickers microhardness is one of the important deciding factors in selecting the processing (cutting,

12 164 grinding and polishing) steps for bulk crystals during the fabrication of devices. Interpretation of hardness is perceived as ability of a material to resist permanent deformation. Microhardness measurement was done with using square based pyramidal diamond indenter having angle 136 between opposite faces. The Vicker s hardness number HV was calculated from the equation (7.3), Where "P" is the applied load in kg/mm 2 and "d" is the average diagonal length of the impression observed in mm. It reveals that both linear and nonlinear patterns were observed. The reason for this behavior is due to bond strength. The nonlinear behavior of the microhardness of the crystal may be due to cleavage plane of the sample. Hardness value of crystal differs from one plane to another which confirms the microhardness anisotropy. The ratio of difference in maximum hardness of two planes to the maximum value of microhardness of the crystal gives the anisotropy coefficient "A". This is given by the equation, A (7.4) To confirm the degree of hardness of the material, a graph has been plotted between log P Vs log d. The relation connecting the applied load (P) and diagonal length (d) of the indenter is given by the Mayer Law P = a d n (7.5) Where "n" is the Mayer index or work hardening coefficient and "a" is the constant for the material. The slope of the straight line by least square fit method gives Mayer index or work hardening coefficients.

13 RESULT AND DISCUSSION Vickers Microhardness Analysis on Grown Crystals Sulphuric Acid Doped Thiourea Single Crystals Table 7.2 Vickers Hardness number (HV) of sulphuric acid doped Load (P) (grm) thiourea Sulphuric acid doped thiourea Length(d) (mm) HV Log P Log d Figure 7.7 A plot load (P) Vs HV for sulphuric acid doped thiourea

14 166 Figure 7.8 A plot log (P) Vs log (d) for sulphuric acid doped thiourea Work hardening coefficient (or) Mayer index number as "n" = Nitric Acid Doped Thiourea Single Crystals Table 7.3 Vickers Hardness number (HV) of nitric acid doped thiourea Load (P) (grm) Length(d) (mm) Nitric acid doped thiourea HV Log P Log d

15 167 Figure 7.9 A plot load (P) Vs HV for nitric acid doped thiourea Figure 7.10 A plot log (P) Vs log (d) for nitric acid doped thiourea Work hardening coefficient (or) Mayer index number as "n" =

16 Sodium Thiourea Chloride Single Crystals Table 7.4 Vickers Hardness number (HV) of sodium thiourea chloride Load (P) (grm) Sodium Thiourea Chloride (NTC) Length(d) (mm) HV Log P Log d Figure 7.11A plot load (P) Vs HV for sodium thiourea chloride

17 169 Figure 7.12 A plot log (P) Vs log (d) for sodium thiourea chloride Work hardening coefficient (or) Mayer index number as "n" = Zinc Thiourea Sulphate Single Crystals Table 7.5 Vickers Hardness number (HV) of zinc thiourea sulphate Load (P) (grm) Zinc Thiourea Sulphate (ZTS) Length(d) (mm) HV Log P Log d

18 170 Figure 7.13 A plot load (P) Vs HV for zinc thiourea sulphate Figure 7.14 A plot log (P) Vs log (d) for zinc thiourea sulphate Work hardening coefficient (or) Mayer index number as "n" =

19 Potassium Thiourea Chloride Single Crystals Table 7.6 Vickers Hardness number (HV) of potassium thiourea chloride Load (P) (grm) Potassium Thiourea Chloride (PTC) Length(d) (mm) HV Log P Log d Figure 7.15 A plot load (P) Vs HV for potassium thiourea chloride

20 172 Figure 7.16 A plot log (P) Vs log (d) for potassium thiourea chloride Work hardening coefficient (or) Mayer index number as "n" = Potassium Thiourea Sulphate Single Crystals Table 7.7 Vickers Hardness number (HV) of potassium thiourea sulphate Load (P) (grm) Potassium Thiourea Sulphate (PTS) Length(d) (mm) HV Log P Log d

21 173 Figure 7.17 A plot load (P) Vs HV for potassium thiourea sulphate Figure 7.18 A plot log (P) Vs log (d) for potassium thiourea sulphate Work hardening coefficient (or) Mayer index number as "n" =

22 Microhardness Comparison Charts of Thiourea Based Grown Crystals Figure 7.19 Vickers hardness comparison chart of thiourea based grown crystals Figure 7.20 Mayer index number comparison chart of thiourea based grown crystals

23 Yield Strength and Elastic Stiffness Constant y) Yield as a permanent molecular rearrangement that begins at a y. The yielding process is very material dependent, being related directly to molecular mobility. It is often possible to control the yielding process by optimizing the materials processing in a way that influences mobility. General purpose polystyrene, for instance, is a weak and brittle plastic often credited with giving plastics a reputation for shoddiness that plagued the industry for years. This occurs because polystyrene at room temperature has so little molecular mobility that it experiences brittle fracture at stresses less than those needed to induce yield with its associated ductile flow. But when that same material is blended with rubber particles of suitable size and composition, it becomes so tough that it is used for batting helmets and ultra-durable childrens toys. This magic is done by control of the yielding process. Yield control to balance strength against toughness is one of the most important aspects of materials engineering for structural applications, and all engineers should be aware of the possibilities. Another important reason for understanding yield is more prosaic: if the material is not allowed to yield, it is not likely to fail. This is not true of brittle materials such as ceramics that fracture before they yield, but in most of the tougher structural materials no damage occurs before yield. It is common design practice to size the structure so as to keep the stresses in the elastic range, short of yield by a suitable safety factor. We therefore need to be able to predict when yielding will occur in general multidimensional stress y.

24 176 Early workers naturally sought an atomistic treatment of the yield process as well. This turned out to be a much more subtle problem than might have been anticipated, and required hypothesizing a type of crystalline defect the "dislocation" to explain the experimentally observed results. Dislocation theory permits a valuable intuitive understanding of yielding in crystalline materials, and explains how yielding can be controlled by alloying and heat treatment. It is one of the principal triumphs of the last century of materials science Stiffness Constant (C 11 ) It is important to distinguish stiffness, which is a measure of the load needed to induce a given deformation in the material, from the strength, which usually refers to the material's resistance to failure by fracture or excessive deformation. The stiffness is usually measured by applying relatively small loads, well short of fracture, and measuring the resulting deformation. Since the deformations in most materials are very small for these loading conditions, the experimental problem is largely one of measuring small changes in length accurately. Hooke made a number of such measurements on long wires under various loads, and observed that to a good approximation the load P and its sufficiently small. This relation, generally known as Hooke's Law, can be written algebraically as (7.6) Where k is a constant of proportionality called the stiffness and having units of lb/in or N/m.

25 177 The stiffness as defined by k is not a function of the material alone, but is also influenced by the specimen shape Yield Strength and Elastic Stiffness Constant of Thiourea Based Grown Crystals The microhardness value correlates with other mechanical y). Yield strength is one of the important properties for device fabrication which can be calculated using the relation y = [HV / 2.9]{[(1-(2-n))][12.5(2-n) / (1-(2-n))] 2-n } (7.7) Where HV is the hardness number and n is microhardening index. The elastic stiffness constant (C 11 ) were calculated for the grown crystals using Wooster`s empirical relation as C 11 = (HV) 7/4 y and stiffness constant (C 11 ) were calculated for grown crystals and are given in the following tables. Table 7.8 Yield strength ( y ) and Stiffness constant (C 11 ) of the Sulphuric acid doped thiourea Sulphuric acid doped thiourea Load (P) HV y MPa C Pa

26 178 Table 7.9 Yield strength ( y ) and Stiffness constant (C 11 ) of the Nitric acid doped thiourea Nitric acid doped thiourea Load (P) HV y MPa C Pa Table 7.10 Yield strength ( y ) and Stiffness constant (C 11 ) of the Sodium thiourea chloride (NTC) Sodium thiourea chloride (NTC) Load (P) HV y MPa C Pa Table 7.11 Yield strength ( y ) and Stiffness constant (C 11 ) of the Zinc thiourea sulphate (ZTS) Zinc thiourea sulphate (ZTS) Load (P) HV y MPa C Pa

27 179 Table 7.12 Yield strength ( y ) and Stiffness constant (C 11 ) of the Potassium thiourea chloride (PTC) Potassium thiourea chloride (PTC) Load (P) HV y MPa C Pa Table 7.13 Yield strength ( y ) and Stiffness constant (C 11 ) of the Potassium thiourea sulphate (PTS) Potassium thiourea sulphate (PTS) Load (P) HV y MPa C Pa CONCLUSION Microhardness studies were carried out in the thiourea based organic and semi-organic crystals at room temperature using microhardness tester, fitted with a diamond pyramidal indenter attached to an incident light microscope. Vickers microhardness values have been calculated and hardness comparison chart have been plotted. The slope of the straight line by least square fit method gives mayer index number / work hardening coefficient as

28 , for sulphuric acid and nitric acid doped thiourea crystals, for NTC, for ZTS, for PTC and for PTS crystals. The result reveals that the materials are soft materials. Yield strength and elastic stiffness constant were calculated for the grown crystals.