Synthesis of $B _4C$ thin films by plasma-enhanced magnetron sputterin

Plazma-destekli manyetik alanda sıçratma tekniğiyle $B _4C$ ince film üretimi

Abstract

Boron carbide (B4C) is the third hardest material at room temperature, which has many other attractive properties such as good wear resistance, high modulus, high chemical and thermal stability. These properties make böron carbide a promising candidate as hard and protective coating for cutting tools, automobile parts, hard disk drives and other wear-resistant applications.Within the range of 8-20% of carbon, boron carbide is ideally described by a rhombohedral unit cell with icosahedral arrangement of 12 atoms, in addition to a three-atom chain along the crystallographic c-axis that interconnect the icosahedra. For amorphous boron carbide fûms deposited by sputtering, it is believed that the structure is still based on a random icosahedral network at a carbon content less than 50%.Several techniques have been used to synthesize boron carbide thin films, including chemical vapor deposition, plasma-enhanced chemical vapor deposition, cathodic arc, atmospheric plasma spraying, electromagnetically accelerated plasma spraying, RF magnetron sputtering, and DC magnetron sputtering. Among these, magnetron-sputtering techniques have been successfully commercialized in a large scale because of their high fdm-deposition rate and low-temperature features.In this study, boron carbide powders were obtained from the carbothermal reduction of boric acid in a graphite resistance furnace at 2000 °C. The powder thus obtained was hot pressed in pure nitrogen atmosphere with 100 MPa applied force at 2100 °C for 15 minutes to obtain boron carbide target used in this study. The sputtering target was 15 cm in diameter and 7 mm in thickness.Boron carbide thin fdms were deposited by plasma-enhanced DC magnetron sputtering of hot-pressed boron carbide target. AISIM2 steel and Si (100) wafers were used as substrates in each deposition. High-purity (99.999%) Ar was used as precursor and was introduced into the vacuum chamber through a mass flow controller to establish the de- sired working pressure, which was 0.3 Pa. The cathode power was fixed at 500 W for all the experiments.Microstructural examinations revealed the presence of continuous and homogeneous B4C films with 350-400 nm thickness.Elemental composition of the films was measured by EPMA. Grazing-angle XRD of the coatings over the total range of process parameters showed no characteristic peaks for boron carbide, indicating that the coatings were amorphous.Infrared spectra presented two broad bands, one centered at ~1100 cm' and the other at ~1570 cm'1. The band at 1100 cm1 is attributed to B-C bonds in the icosahedra and is characteristic of B4C thin film structure. The band at 1570 cm'1 has been attributed either to the presence of graphite or free carbon in boron carbide structure or to the stretching in the linear chains that interconnect the icosahedra.Nanoindentation results demonstrated that boron carbide films deposited are remarkably hard and the increase in the negative bias voltage led to an increase in the measured hardness from 32 GPa for the film deposited at floating potential to a maximum value of -40 GPa for the coating deposited at 100 V bias voltage. Further increase in the bias voltage to 250 V resulted with a decrease in the hardness to 32 GPa. The same tendency of increase was observed for the Young's modulus, from 270 GPa for the films deposited at floating potential, it reached its maximum value for 300 GPa at 100 V bias voltage and then decreased thereafter to 265 GPa for 250 V bias voltage.Modified sputtering configuration led non-columnar, featureless microstructures with smooth surface morphologies. There was no significant effect of the deposition parameters on the thickness and elemental composition of the films deposited. Films were amorphous and exhibited remarkably high hardness and Young's modulus values with high elasticity. With the increase in the bias voltage, increases in the hardness and Young's modulus of boron carbide films were observed.

Özet

Bor karbür, yüksek sertliği, mekanik, tribolojik, elektronik, optik özelliklerinin yanı sıra yüksek nötron absorblama özelliği ile de dikkat çeken bir malzemedir. Düşük yoğunluğu, yüksek Young modülü, çok yüksek termal ve kimyasal kararlılığı vb. özellikleri olan B4C, elmas ve c-BN'den sonra bilinen en sert malzeme olmasına karşın bu durum oda sıcaklığında geçerlidir. Elmas ve c-BN'ün sertliği artan sıcaklıkla kademeli olarak düşerken B4C termal kararlılığı sayesinde yüksek sıcaklıklarda sertliğini muhafaza etmektedir ve özellikle 1100 C'nin üzerindeki sıcaklıklarda bilinen en sert malzemedir. Bu özellikleriyle B4C ince filmler, kesici takımların, fren balatalarının, sabit disklerin ve çeşitli makine parçalarının kaplanması gibi mekanik, tribolojik uygulamalarm yanı sıra, yüksek sıcaklık ortamları gibi zorlayıcı şartlarda çalışacak transistörler vb. elektronik ve optik uygulamalarda da kullanılmaktadır. Bu çalışmada, bor karbür tozlarının sıcak preslenmesiyle elde edilmiş olan bor karbür hedef malzeme kullanılarak, plazma-destekli doğru akım manyetik alan sıçratma tekniğiyle 350-400 nm kalınlığında homojen ve taban malzemeye iyi yapışan bor karbür ince filmler üretilmiştir. Biriktirme sıcaklığı tüm kaplamalar için 250 C olarak sabitlenmiş ve 0-250 V arasında uygulanan taban malzeme voltajının kaplama yapısına olan etkileri incelenmiştir. Elektron sondası mikro analizleri (EPMA), kaplamaların elementel bileşimlerinin üretim şartlarından bağımsız olduğunu ortaya koymuştur. Kesitten gerçekleştirilen taramalı elektron mikroskobu (SEM) incelemeleri sonucunda, bu çalışma şartlarında bor karbür filmlerin kolonsuz yapıda biriktiği tespit edilmiştir. Nanosertlik testleri neticesinde, taban malzeme voltajının arttırılmasıyla sertlik ve Young modülü değerlerinde önemli bir artışın meydana geldiği tespit edilmiştir.