change one fontface to another fontface in c# wpf

I want to change one LTibetan fontface to Arial black fontface in c# WPF but rest Fontfaces are remains same as. I have used below code to achieve the purpose:So anybody kindly give me a help

void ChangeFontFamily(object sender, RoutedEventArgs e)

{

FontFamily ffamily = new FontFamily();

ffamily = btn2.FontFamily;

ffamily = richTextBox1.FontFamily;

string str = ffamily.ToString();

if (str == ("LTibetan"))

{

btn2.FontFamily = new FontFamily("Arial Black");

richTextBox1.FontFamily = new FontFamily("Arial Black");

btn2.Content = " Arial Black";

}