From nanocrystalline to amorphous structure in beryllium-based coatings
Introduction
Beryllium (Be) is a metal of major importance in the space and nuclear industries [1], [2]. It has a low density, high elastic modulus, elevated melting point, high heat capacity, and good nuclear hardness. These attributes make Be attractive for space structures, such as optical benches, camera housings, rocket nozzles, and optical mirrors [3], [4]. For example, Be is ideal for space mirror sensors because of its low density and high micro-yield strength (i.e. the stress that gives rise to a plastic strain of 10−6) [5]. The high micro-yield strength is a requirement for Be components to remain undistorted during fast space maneuvers. However, Be also suffers from some intrinsic problems such as low ductility and poor manufacturability, not to mention its toxicity, that limits it from broad applications. It’s generally accepted that the low ductility is associated with its hexagonal close packed (hcp) structure, propensity for localized slip, and high impurity content (especially BeO). Previous efforts have been made to improve ductility either by alloying to change the crystal structure from hcp to body centered cubic (bcc) or by forming intermetallic beryllides [6], [7]. However, these approaches have met with little success. Furthermore, although some binary and ternary phase diagrams were developed as a result of these efforts, general information on the phase relationships between Be and other elements is extremely limited.
There is a requirement for hollow spheres of Be having uniform wall thickness for use as ablative fuel containers in plasma physics studies of inertial confinement fusion [8]. The nearly pure Be spheres are selectively doped with a heavier element such as Cu (at about 0.9 at.%) in order to adjust the opacity related to x-ray absorption effects. The performance advantages of Be targets, however, are accompanied by demanding material requirements. For example, Be offers the fundamental potential for improved performance as an ablating material since it has lower opacity, a larger ablation rate, more initial mass, and higher bulk strength than polymeric counterparts. Problems arise in practice, however, in a typical target design, a thin-walled spherical mandrel of a polymer is coated with a smooth, strong and thick Be-rich alloy [8], [9]. As the Be is coated, using a vapor deposition process, the corresponding textured and crystalline columnar growth produces an intrinsic roughness in the capsule surface. Such rough surfaces can lead to damaging Rayleigh-Taylor instabilities during the ablation of the Be coating [8]. This induced instability degrades the compression of the fuel within the capsule which in turn cools the fuel. To minimize such surface roughness effects down to the nanometer scale [9], [10], either the conventional vibration-levitation coating process (that uses bounce pans to coat Be onto mandrels) must accommodate in situ smoothing or to use a post-deposition tool (such as a high-voltage clustered ion beam).
Alternative methods have been developed to minimize surface roughness effects during the sputter deposition of Be [10], [11], [12], [13]. For example, the application of a negative bias potential to the substrate holder induces in situ bombardment of the capsule that results in the reduction of surface roughness [10], [12]. Because the surface roughness is found to scale with the width of the columnar deposit, i.e. the grain size, a concurrent increase in strength is measured for the coating; this strength-grain size relationship follows a typical Hall-Petch relationship [12]. As a result, the ultimate strength increases three-fold from a value of less than 50 MPa for a non-biased deposition to greater than 150 MPa for a negative high-biased deposition. This approach to minimizing surface roughness, however, appears not to be effective below a lower bound value of 40 nm rms. In another attempt to reduce surface roughness, induced by particulates from the open-bounce pan configuration that is typically used to coat small capsules, a chambered design decreased the comparative roughness of thin Al coatings by a factor of three or more [13]. This approach, alone or in combination with the application of ion bombardment, remain insufficient to reduce surface roughness to the nanoscale level.
The hardness of crystalline films is not only related to grain size but also the chemical bonding within the material. The chemical bonding features of B and its nitride phases are revealed by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy [14], [15]. These chemical bonding features are known to correlate with the hardness of crystalline BN films and amorphous B films [16], [17], [18]. Of particular interest, with regards to creating smooth Be coatings, is the sputter deposition of B since it has a similar density and molecular weight. Sputter deposition of a pure B target produces a dense and hard coating that is homogeneous in its amorphous, as-deposited, state [16], [17]. Therefore, co-deposition of Be with B is a possible approach to reduce roughness, improve homogeneity, and strength through the refinement of grain size to the nanoscale. This is because the alloying of B with Be could lead to glassy phase formation for B compositions greater than that of the proposed 11 at.% B eutectic composition found in the binary Be-B alloy phase diagram [19]. Often, deep eutectics indicate that the structure of a liquid, i.e. an amorphous phase, can be stabilized at low temperature. The rapid quench that is needed to stabilize such glassy solid phases from the melt, or from a gaseous phase, can be readily accomplished through sputter deposition process [20]. In addition, it is known from studies of evaporation and sputter deposition that the grain size of nominally pure (i.e. 99.8 at.%) Be can be dramatically refined through the incorporation of impurities (e.g. Al and Fe) [21] at low concentrations and refractory metals (e.g. Ti and Zr) [22], [23], [24] at increased concentrations. Furthermore, little is known about the Be-rich end of the phase diagrams in these glass-forming systems. In total, these prior findings have led to the present study in which the composition effects of select metals such as B, Cu, and Fe on the microstructure and mechanical properties of Be-based coatings. The emphasis will be focused on B, since it is known to be amorphous in the sputter deposited condition, i.e. it is atomically smooth.
Section snippets
Experimental
Be-based coatings were synthesized by sputter deposition onto polished silicon wafers using planar magnetrons. The deposition chamber contains a circular array of three 3.3 cm diameter planar magnetrons. Each magnetron source is rate calibrated for its target material and used in concert to create a desired composition for the coating. The center of each sputter source is located along the circumference of a 7 cm diameter circle at a 120° separation. The sputter targets are fully dense powder
Results
The Be and alloy coatings produced for this study are summarized in Table 1. The microstructure of the crystalline Be and alloy coatings are revealed by bright field imaging of samples in plan view (as seen in Fig. 1a–d) and under a 30° tilt (in Fig. 1e–h). The growth direction of each sample is displayed from the bottom to the top of each micrograph. Measurements of lattice plane spacing from the selected area diffraction patterns (SADPs) of these Be-rich coatings are best fit to the hcp
Analysis and discussion
The uniform control of composition is especially important in applications of Be coatings for inertial confinement fusion targets [8]. Smooth Be-based coatings must be uniformly deposited on hollow spherical capsules. In one approach, the capsules randomly bounce and collide with one another in an ultrasonically-vibrated bounce pan [9]. In an alternate approach for greater control of motion, each capsule is tumbled within an individual chamber about twice the size of the mandrel diameter [16].
Summary
Beryllium and binary Be alloy coatings have been prepared for the purpose of refining grain size down to the nanoscale range through the method of sputter deposition. The coatings have been characterized using electron microscopy and nanoindentation testing. The addition of small concentrations of metals such as Fe and B is very effective at refining the width of the as-deposited columnar structure to less than 20 nm in size. The use of B is found to stabilize an amorphous binary phase of Be
Acknowledgements
The authors thank Phil Ramsey for contributions to the synthesis of the coatings, to Bill Choi for contributions to the hardness measurements, and to Christoph Bostedt for contributions to the near-edge x-ray absorption fine structure measurements that were conducted on Beamline 8.0.1 at the Advanced Light Source in Berkeley. This work was performed under the auspices of the US Department of Energy by University of California, Lawrence Livermore National Laboratory under contract No.
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