How to replace the search submit button with a Font Awesome character?
In order to add the new form button, I used hook_form_FORM_ID_alter in my template.theme file in my theme folder (Notice:replace ThemeName with the name of your theme in below code):

ThemeName.theme:

**
 * Replace the Search Submit Button with a Font Awesome Character.
 */
function ThemeName_form_search_block_form_alter(&amp;$form, &amp;$form_state) {
  $form['keys']['#attributes']['placeholder'][] = t('Search');
  $form['actions']['submit']['#value'] = html_entity_decode('&amp;#xf002;');
  $form['actions']['submit']['#attributes']['class'][] = 'search-button';
}


Remeber use html_entity_decode() to parse the Unicode code.
and CSS:

.search-button input[type="submit"]{
    font-family: FontAwesome;
    color: #fff !important;
    display: inline;
    font-size: 18px;
}