Bal-tec™ Home Tantalum Marker Bead Micro Hardness Testing
It is hard enough to just test the hardness of miniature balls when they are made of conventional materials, but measuring soft as putty tantalum balls that may be as small as 0.010 inch ( .25mm ) diameter is a real test of both facilities and personnel. The pure tantalum metal that is used in the manufacture of radiographic marker beads is highly ductile and very soft. This means that the hardness testing machine must apply a very light load to the indenter, limiting its penetration into the tantalum to a reliable level. The choice of the hardness test method to be used is determined primarily by the miniature size of the radiographic marker beads. The size of radiographic marker beads generally range from one point two millimeter (1.2 mm, 0.049”) diameter down to zero point two five millimeters (0.25 mm, 0.010”) diameter.
All of this size range falls in the realm of Micro Hardness measuring. The two conventional micro hardness techniques are the Vickers hardness test and the Knoop hardness test. The choice between the two methods is a black and white decision. The knoop diamond indenter has four prismatic facets that have very flat angles. These large flat facets spread out the load so it doesn’t penetrate the putty soft tantalum, nearly as deep as the steep pyramidal indenter of the Vickers test would. This dramatically improves the accuracy and the repeatability of the Knoop hardness measurement. The micro hardness test machine must be mounted on a vibration isolating platform. Any vibration, but especially any low frequency vibration, will reduce the apparent hardness of the test specimen. Vibration is a much more serious problem when measuring soft materials. The fall time and the dwell time of the diamond indenter must be accurately controlled. The timing watch used to monitor these parameters must be accurately calibrated. A bargain store stop watch is not a satisfactory device.
The fall time is set by adjusting the dash pot of the hardness test machine. The fall time will change with any change in the temperature of the room, so it is important that it be checked frequently. The dwell time is measured with a calibrated watch. The dead weights used to apply the indenting force to the diamond tool must be certified. Warning: The weights are typically marked with the force actually applied to the test part by the hardness measuring machine, not the mass of the weight itself. The actual mass of the test weights, manufactured by the very same company, for different models of their own machines may vary dramatically, so they may not be used interchangeably. Even individual weights, used on the same hardness test machine, may not have the same ratio of mass. In one case, the physical shape of the dead weight for one scale, was dramatically off its geometric centerline. This off centered mass changed the position of the mass on the fulcrum of the machine, which required an overall reduction in the mass of this one test weight.
Usually the pivoting beam of the micro hardness test machine is exactly balanced without the diamond indenter installed. The value marked on the dead weight will be that of the weight itself and of the diamond indenter combined. Certifying the actual force applied by the point of the diamond indenter to the test part can be done to a high degree of accuracy with relative ease. A simple laboratory triple beam balance scale adapted so that the weighing pan is under the diamond indenter can be used to indicate the true force applied to the indenter. To be accurate, the force must be measured with the diamond indenter installed. To eliminate off axis loading of the scale pan by the diamond indenter, a piece of rubber is placed on top of the scale pan. The validity of this technique has been verified by substituting a very accurate hardened steel ball for the diamond indenter. This ball has the exact same mass as the diamond indenter itself. This ball is used as a self aligning isolator between the hardness tester and the pan of the scale.