Niobium Sheet for the Synthetic Diamond Industry
Courtesy of Haliburton
The niobium crucibles are produced from sheet material by deep drawing. The quality and surface finish of these crucibles is critical to the final quality of the synthetic diamonds. To meet the deep draw challenges and quality requirements for synthetic diamonds, H.C. Starck’s development engineers made significant improvements to our niobium sheet that addresses the challenges and issues inherent to manufacturing the crucibles.
Niobium Sheet Deep Drawn to Form Crucibles
The niobium crucible is a single-use container designed for the high pressure and high temperature (HPHT) process in making synthetic diamonds. The production of these crucibles starts with relatively simple and straightforward cold rolling and annealing processes to produce niobium sheet material. The niobium sheet is deep drawn into crucibles through a punch process that uses one or more dies to manufacture the small crucibles.
One of the key advantages of H.C. Starck’s niobium is our ability to roll sheet evenly, so it does not become thin, crack, or fail during the deep drawing process. In order for the niobium sheet to withstand multi-axial tensile stresses without failure, it has to be extremely ductile and possess suitable anisotropic mechanical properties to reduce wall-thinning during deep drawing.
Anisotropy in sheet is gauged by a number, called the r-value (or Lankford coefficient), which refers to the ratio of true strain along the width to true strain along the thickness during a tensile test.
Avoiding Crucible Defects with Niobium Sheet
There are many variables to consider when designing a deep draw process. An incorrect setting on any of the steps can lead to defects in the final deep drawn component.
Wrinkling in the flange occurs due to low blank holding force.
Wrinkling in the wall occurs when a wrinkled flange is drawn into the cup or clearances are very large.
Tearing occurs from high tensile stresses that cause sheet thinning. Tearing also occurs from sharp corner radii, high length-to-diameter (L/D) ratios, high pressure pad loads, and high punch loads.
Earring occurs when the material is anisotropic (i.e., directionality in properties)
Surface scratches occur if the punch and die are not smooth or if process lubrication is inadequate.
Radial cracks in the flanges and edge of the cup due to insufficient metal ductility.
Orange peel (surface roughness) occurs in coarse grain metals.
The most common material related defect issue is “orange peel” because of the coarse grain sizes. A related niobium microstructure can show poor flatness and smoothness of the deep drawn cup as each grain tends to deform independently and non-uniformly. Poor flatness and/or smoothness can produce a synthetic diamond that requires excessive grinding. A coarse grain structure can also cause tearing of the sheet during deep draw operations.
Grain Stabilized Niobium (GSNb)
H.C. Starck’s grain stabilized niobium sheet is a single-phase micro-alloyed niobium material that has a grain size of approximately 2 ASTM numbers finer than commercial grade niobium. This reduces the “orange peel” affect during drawing and forming operations and is particularly well suited for deep draw applications where uniform deformation is required.
GSNb is used extensively in the industrial diamond market. Additionally it has corrosion resistant qualities equal to commercial grade niobium for applications in the chemical process industry. GSNb can also be used in sputtering targets for fiber optic applications and architectural glass. In nuclear reactors it has low thermal neutron cross-section and superior corrosion resistance. It is an excellent getter and finds use in high temperature vacuum furnaces, and is resistant to attack by the molten alkali metals found in sodium vapour lamps.
Optimizing Deep Drawing Capabilities
Deep drawing capabilities of niobium sheet are impacted by the amount of silicon (Si) added to the niobium composition. The effect of the Si concentration on the r-value or drawability in niobium sheet has been studied in detail. Thick sheets of 0.01” (254 µm) with Si concentrations between 40-135 ppm were measured in tensile testing, and it was found that the average r-value decreases linearly as the Si concentration increases. A value of less than 1.0 can be obtained when the Si concentration is greater than 85 ppm. Additionally, it was discovered that the Si concentration has its maximum effect on r-0 degree and r-90 degree texture measurements, and it is proposed that Si atoms may be pinning grains with <100>//RD and thereby, effecting the r-value.
For best deep drawing characteristics, an optimal Si concentration of 85 ppm was determined to ensure that the mean r-value remains greater than 1.0 while also maintaining a small grain size. Good control of the Si concentration is critical to obtain consistent performance during deep drawing.
The corrosion resistance of GSNb is identical to that of commercial grade niobium and can be used in all applications where commercial grade niobium is used. Like tantalum, GSNb is resistant to most acids with the exception of hydrofluoric, however it is not as resistant as tantalum to strong acids at high temperature. It should not be used with strong bases (alkalis).
For further product specifications please download the product brochure below.