Pengaruh Aditif Silikon Dioksida dan Post-Alkali Treatment Terhadap Nilai Spesifik Abrasi, Kekerasan, dan Korosi Lapisan Hasil Plasma Electrolytic Oxidation Pada Logam Magnesium dan Titanium = The Effect of Silicon Dioxide Additives and Post-Alkali Treatment on Spesific Abration, Hardness, and Corrosion of Plasma Electrolytic Oxidation Layers on Magnesium and Titanium Metals