Section 1: Introduction to Hardness Measurement Techniques Section 2: Micro Hardness Measurement Procedure Using HXD-1000TMC

Section 1: Introduction to Hardness measurement techniques Section 2: Micro Hardness measurement Procedure using HXD-1000TMC

Section 1

Introduction

Most of the books define hardness as “Resistance of metal to plastic deformation”, usually indentation. In general, hardness usually implies a resistance to deformation. However, the term may also refer to stiffness or temper or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently deformed (bent or broken), when a load is applied. In mineralogy, hardness is defined as the resistance of the substance to being scratched by another substance. In metallurgy hardness is defined as the ability of a material to resist plastic deformation.

Hardness Measurement Scales

Hardness measurement can be on macro, micro and nano scale according to the forces applied and deformation obtained. Macro-hardness measurement is a simple method of obtaining mechanical property data from the bulk material from a small sample. When concerned with coatings and surface properties of importance to friction and wear processes for instance, the macro indentation depth would be too large relative to the surface scale features. Micro hardness measurements are appropriate, where materials have a fine microstructure, are multi-phase, non homogeneous or prone to cracking, where as the macro hardness measurements are highly variable. Micro hardness is determined by forcing an indenter such as Vickers or Knoop indenter into the surface of the material under 15 to 1000gf load. Usually the indents are so small that they have to be measured with a microscope. Nano indentation tests measure hardness by indenting using very small indentation forces, on the order of 1nano-Newton and measuring the depth of the indention made.

Hardness Measurement Methods

Metals: There are three general types of hardness measurements depending on the manner of test conducted. These are: 1.) Scratch hardness 2.) Indentation hardness 3.) Rebound or dynamic hardness Of all of these, only indentation hardness is of major engineering interest for metals.

1 1.) Scratch Hardness This technique is of major interest for mineralogists. This measure of hardness, various minerals and other materials are rated on their ability to scratch one another. Scratch hardness is measured according to the Moh’s scale. This consists of 10 standard minerals arranged in the order of their ability to be scratched. The Moh’s scale is not well suited for metals since the intervals are not widely spaced in the high-hardness range.

2.) Indentation Hardness There are three types of tests used: A.) Rockwell hardness test B.) Brinell hardness test C.) Vickers hardness test. These hardness tests determine the metal’s resistance to the penetration of a non- deformable ball or cone.

A.) Rockwell Hardness Test This test is a hardness measurement based on the depth of indentation, under constant load, as a measure of hardness. A minor load of 10Kg is first applied to seat the specimen. This minimizes the amount of surface preparation. The major load is then applied, and the depth of indentation is automatically recorded on a dial gage in terms of arbitrary hardness numbers. The dial contains 100 divisions, each division representing a penetration of 0.002mm. A 120º diamond cone with a slightly rounded point, called a Brale indenter , 1.6mm and 3.2mm diameter steel balls are generally used as indenters. Major loads of 60, 100, 150Kg are used. Since it is dependent on load and indenter, it is necessary to specify the combination which is used. This is done by prefixing the hardness number with a letter indicating the particular combination of load and indenter for the hardness scale employed. A Rockwell hardness number without the letter prefix is meaningless. The following are standardized set of scales set by ASTM (American Society of Testing and Materials):

• A-Cemented carbides , thin steel and shallow case hardened steel • B-Copper alloys, soft steels, aluminum alloys, malleable iron, etc • C-Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel and other materials harder that B 100. • D- Thin steel and medium case hardened steel and pearlitic malleable iron. • E-Cast iron and aluminum and magnesium alloys, bearing metals. • F-Annealed copper alloys, thin soft sheet metals. • G-phosphor bronze, beryllium copper, malleable irons. • H-Aluminum, zinc and lead. • K, L, M, P, R, S, V- Baring metals and other very soft or thin materials including plastics.

B.) Brinell Hardness Test Brinell hardness is determined by forcing a hard steel or carbide sphere of a specified diameter under a specified load onto the surface of the material and measuring the diameter of the indentation left after the test.

2 The Brinell hardness test consists in indenting the metal surface with a 10mm diameter steel ball at a load of 3000Kg. For soft metals the load is reduced to 500Kg to avoid too deep an impression, and for very hard metals tungsten carbide ball is used to minimize distortion of the indenter. The load is applied for a standard time, usually 30seconds and the diameter of the indentation is measured with a low power microscope after removal of the load. The Brinell hardness number is obtained by dividing the load used, in kilograms, by the actual surface area in the indentation, in square millimeters. This is expressed in the following formula ( figure 1 ):

Figure 1

Where, BHN= the Brinell hardness number F= the imposed load in Kg D= the diameter of the spherical indenter in mm d = diameter of the resulting indenter impression in mm

C.) Vickers Hardness Test: This hardness test uses a square base diamond pyramid as the indenter. The opposite sides meet at the apex at an angle of 136º. The angle was chosen because it approximates the most desirable ratio of indentation diameter to ball diameter in the Brinell hardness test. Because of the shape of the indenter, this is frequently called the diamond pyramid hardness test. The diamond pyramid hardness number (DPH), or Vickers hardness number (VPH), is defined as the load divide by the surface area of the indentation. The diamond is pressed into the surface of the materials at loads ranging upto approximately 120Kg force, and the size of the impression (usually no more than 0.5mm) is measured with the aid of a calibrated microscope. The Vickers number (HV) is calculated using the formula:

3 2Psin (θ 2/ ) .1 854 P VHN = = L2 L2 Where, P = Applied load in Kg D = Average length of the diagonal of the square shaped indent produced θ = Angle between the opposite faces of the diamond = 136 o A perfect indentation made with a perfect diamond-pyramid indenter would be a square. Figure 2 shows how the diamond indenter looks and the indentation made on the surface.

Figure-2

3.) Rebound or Dynamic Hardness: Shore hardness: The shore scleroscope measures hardness in terms of the elasticity of the material. A diamond tipped hammer in a graduated glass tube is allowed to fall from a known height on the specimen to be tested and the hardness number depends on the height to which the hammer rebounds. The harder the material the higher the rebound.

Hardness testing for ceramics: In engineering most of the ceramic hardness values are of Vickers, with loads ranging from few Newton’s to 9.8N(1Kgf) and for high toughness ceramics the load could go up to 98N(10Kgf). Knoop hardness is also used for measuring the hardness for ceramics whose loads would range from 0.98N(100gf) to 19.6N(2Kgf). Since the indentation loads are very low in ceramics, problems arise from uncertainty measurement and if the loads are higher then it might result in cracking or spalling. So it is very important to select the suitable load for ceramic hardness testing.

4 Section 2: Experimental Procedure

Micro Hardness measurement procedure using HXD-1000TMC

Precautions to be followed before starting the experiment: • First important precaution is to handle the instrument very gently as to maintain the sensitivity of the instrument. • While performing the test the instrument should be free from any shock or vibration. • The indenter anvil should be clean and well seated ( figure 3 ). • Never touch the indenter while aligning the sample. • The surface to be tested should be polished, clean, dry and smooth. Rough and irregular surface with dirt will damage the hardness testing machine. • The surface should be flat and perpendicular to the indenter, the perpendicularity and the flatness depends on the sample flatness. Make sure the sample is flat before it sits on the stage. • The thickness of the specimen should be such that a mark or bulge is not produced on the reverse side of the piece. • The spacing between indentations should be five to eight times the diameter of the indentation. • Make sure the thickness of the sample is less than 3/4 th of an inch.

Microhardness Tester HXD-1000TMC

Load Knob

X-Y Stage Manipulators

Focus Wheel

Power Timer Button Start

Figure 1(Front view of HXD-1000TMC) Figure 2 (Side view of HXD-1000TMC)

Microscope Objective

Anvil

Indenter

Sample Lever

Figure 3(Stage of HXD-1000TMC)

Technical specifications: Range of hardness measurements- 5 HV-3000 HV Objective- 40X, 10X Eye piece-15X Load Range-10g-1000g

Experiment Procedure:

• Switch on the power button on the power stabilizer as shown in figure1 . • Switch on the red button which is at the back of the instrument. • Place the sample on the stage with the help of the holders and the adjustable lever as shown in figure 3 . • Place the sample using the manipulators to the desired location where the optical microscope is focused as shown in figure 1 . • Open the microhardness test software in the computer. As soon as the software is opened it displays as shown in figure 4 .

Figure 4

• Focus the sample by adjusting the focus wheel as shown in figure 2. Figure 5 shows an example when the sample is in focus.

Figure 5

• Set the load using the load knob as shown in figure2 . Selecting the load depends on type of sample i.e. metal or ceramic. For soft metals, load can be around 100- 200 gm. For hard metals load could be 300-500 gm. For ceramics, load between 50-200 gm is desirable. • Set the dwell time using the timer button mentioned in figure 1. Dwell time should be selected as 15 seconds. • Select the area where the measurement required by adjusting the x-y motion of the stage and make sure the sample is in focus which can be observed live on the computer screen. • Left click test parameters and set the force/load and dwell time, they should match with the values input in the instrument before. • Press start button as shown in figure 1 and stay away from the indenter and objective lens. The turret will automatically move and make an indentation onf the selected area. After the indentation is made, turret will automatically move and indent will be displayed on computer screen as shown in figure 6.

Figure 6

• Now left click on analyze, software will automatically generate the boundaries of the diagonals. If these are accurately fitting the edges then hit ok on the pop up window otherwise click cancel.

HV Rea ding

Figure 7

• Right click on the diagonals and automatically the software will give the diagonal lengths and HV value on the top left corner of the screen as shown in figure 7. • This screen shot can be saved by clicking on the save button. • Repeat the whole process and collect 8 readings at different locations (each location should be approximately 8 times the diagonal distance of each indent) and take the mean value as the hardness number and the standard deviation is the error generated while taking the readings.

This experiment should be repeated for 2 different samples:

Sample A: Steel sample

Sample B: Aluminum sample

Report should include following: observations data reporting statistical analysis representative picture of indent for each sample conclusion.

NOTE: You should not leave the experimental data on computer’s hard drive. Copy it in a flash drive at the end of experiment. Any data stored on hard drive will be erased next day and you may have to repeat the experiment if you will not copy the data.